DTX DL: Add FACCH cache

When DL DTX is active and silent period is in progress dtx.cache is
populated by SID UPDATE message which about to be scheduled next. If at
that moment FACCH message arrives (which have higher priority) we have
to send ONSET message to L1 but we can't invalidate cache with SID
UPDATE as it will be used for SID FIRST message to resume silent period
after FACCH transmission is over (provided there were no incoming voice
in between). Hence the necessity for separate buffer to store content of
FACCH message while we're sending ONSET to L1 while keeping SID UPDATE
cached.

Change-Id: I316e81af893b24766bf259baaed7a0be75a11694
Related: OS#1801

openbsc/include/openbsc/gsm_data_shared.h

@@ -300,7 +300,10 @@ struct gsm_lchan {
 		struct amr_multirate_conf amr_mr;
 		struct {
 			struct osmo_fsm_inst *dl_amr_fsm;
+			/* TCH cache */
 			uint8_t cache[20];
+			/* FACCH cache */
+			uint8_t facch[GSM_MACBLOCK_LEN];
 			uint8_t len;
 			uint32_t fn;
 			bool is_update;