Cosmetic: GSUP: note "on wire" for imei_result

Note that OSMO_GSUP_IMEI_RESULT_ACK is 0 on the wire, although the
enum value is 1. Same with NACK (1 on wire, enum 2).

I had implemented enum osmo_gsup_imei_result after
enum osmo_gsup_cancel_type above, where this comment exists as well,
and I incorrectly assumed that enum osmo_gsup_cn_domain in the middle
was also implemented this way and therefore adding the comments to each
enum would be redundant. But for cn_domain, the values on the wire are
the same as the enum values.

Change-Id: If97c34f117bfaab2232bbb625e9d118c8f390e58

include/osmocom/gsm/gsup.h

@@ -174,8 +174,8 @@ enum osmo_gsup_cn_domain {
 };
 
 enum osmo_gsup_imei_result {
-	OSMO_GSUP_IMEI_RESULT_ACK		= 1,
-	OSMO_GSUP_IMEI_RESULT_NACK		= 2,
+	OSMO_GSUP_IMEI_RESULT_ACK		= 1, /* on wire: 0 */
+	OSMO_GSUP_IMEI_RESULT_NACK		= 2, /* on wire: 1 */
 };
 
 /*! TCAP-like session state */