Use gsm48_encode_ra() for RAI encoding

It has stricter type signature which increase the chance of spotting
misuse either via compiler warning or with automated scan. This also
paves the way for gsm48_construct_ra() deprecation in libosmocore.

Change-Id: I2c0f082dc7214ed57a40dad0788e34b838dfac97
Related: OS#1640
diff --git a/src/gprs/gprs_gmm.c b/src/gprs/gprs_gmm.c
index 77a5537..9313e98 100644
--- a/src/gprs/gprs_gmm.c
+++ b/src/gprs/gprs_gmm.c
@@ -448,7 +448,7 @@
 	aa->att_result = 1;	/* GPRS only */
 	aa->ra_upd_timer = gprs_secs_to_tmr_floor(sgsn->cfg.timers.T3312);
 	aa->radio_prio = 4;	/* lowest */
-	gsm48_construct_ra((uint8_t *)&aa->ra_id, &mm->ra);
+	gsm48_encode_ra(&aa->ra_id, &mm->ra);
 
 #if 0
 	/* Optional: P-TMSI signature */
@@ -1505,7 +1505,7 @@
 	rua->upd_result = 0;	/* RA updated */
 	rua->ra_upd_timer = gprs_secs_to_tmr_floor(sgsn->cfg.timers.T3312);
 
-	gsm48_construct_ra((uint8_t *)&rua->ra_id, &mm->ra);
+	gsm48_encode_ra(&rua->ra_id, &mm->ra);
 
 #if 0
 	/* Optional: P-TMSI signature */