csn1: fix: never use enumerated types in codec structures

I faced a problem while working on EGPRS Packet Channel Request
coding support: the unit test I wrote for it was passing when
compiled with AddressSanitizer, but failing when compiled
without it o_O. Somehow this was observed only with GCC 10.

Here is a part the standard output diff for that unit test:

   *** testEGPRSPktChReq ***
   decode_egprs_pkt_ch_req(0x2b5) returns 0
  - ==> One Phase Access
  + ==> unknown 0xdd5f4e00
   decode_egprs_pkt_ch_req(0x14a) returns 0
  - ==> One Phase Access
  + ==> unknown 0xdd5f4e00
   decode_egprs_pkt_ch_req(0x428) returns 0
  - ==> Short Access
  + ==> unknown 0xdd5f4e01

At the same time, debug output of the CSN.1 decoder looked fine.
So WYSINWYG (What You See Is *NOT* What You Get)! As it turned
out, this was happening because I used an enumerated type to
represent the sub-type of EGPRS Packet Channel Request.

  typedef struct
  {
    EGPRS_PacketChannelRequestType_t      Type; // <-- enum
    EGPRS_PacketChannelRequestContent_t	  Content;
  } EGPRS_PacketChannelRequest_t;

The problem is that length of an enumerated field, more precisely
the amount of bytes it takes in the memory, is compiler/machine
dependent. While the CSN.1 decoder assumes that the field holding
sequential number of the chosen element is one octet long, so its
address is getting casted to (guint8 *) and the value is written
to the first MSB.

  // csnStreamDecoder(), case CSN_CHOICE:
  pui8  = pui8DATA(data, pDescr->offset);
  *pui8 = i; // [ --> xx .. .. .. ]

Let's make sure that none of the existing RLC/MAC definitions is
using enumerated types, and add a warning comment to CSN_CHOICE.

Affected CSN.1 definitions (unit test output adjusted):

  - Additional_access_technologies_struct_t,
  - Channel_Request_Description_t.

Change-Id: I917a40647480c6f6f3b0e68674ce9894379a9e7f

src/csn1.h

@@ -493,9 +493,11 @@ gint16 csnStreamEncoder(csnStream_t* ar, const CSN_DESCR* pDescr, struct bitvec
  * The value of the address is called a selector. Up to 256 (UCHAR_MAX) unique
  * selectors can be handled, longer choice list would cause CSN_ERROR_IN_SCRIPT.
  * After unpacking, this value is then converted to the sequential number of the
- * element in the union and stored in the UnionType variable.
+ * element in the union and stored in the UnionType variable (Par2).
  *      Par1: C structure name
- *      Par2: C structure element name
+ *      Par2: C structure field name holding sequential number of the chosen element.
+ *            BEWARE! Never use an enumerated type here, because its length is
+ *            compiler/machine dependent, while decoder would cast it to guint8.
  *      Par3: address of an array of type CSN_ChoiceElement_t where all possible
  *            values of the selector are provided, together with the selector
  *            length expressed in bits and the address of  the CSN_DESCR type

src/gsm_rlcmac.h

@@ -158,7 +158,7 @@ typedef struct
 {
   guint8 PEAK_THROUGHPUT_CLASS;
   guint8 RADIO_PRIORITY;
-  RLC_MODE_t RLC_MODE;
+  guint8 RLC_MODE;
   guint8 LLC_PDU_TYPE;
   guint16 RLC_OCTET_COUNT;
 } Channel_Request_Description_t;
@@ -1245,7 +1245,7 @@ typedef enum
 
 typedef struct
 {
-  AccessTechnology_t Access_Technology_Type;
+  guint8              Access_Technology_Type;
   guint8              GMSK_Power_class;
   guint8              Eight_PSK_Power_class;
 } Additional_access_technologies_struct_t;