Gb: use textual representation for parse log

Use textual representation for message type and protocol descriminator
in case of Gb parsing errors.

Change-Id: Ida925258be119619d8705361730c554a130b75bc
Related: SYS#3610
diff --git a/openbsc/src/gprs/gprs_gb_parse.c b/openbsc/src/gprs/gprs_gb_parse.c
index 9befa39..d5a122b 100644
--- a/openbsc/src/gprs/gprs_gb_parse.c
+++ b/openbsc/src/gprs/gprs_gb_parse.c
@@ -380,9 +380,8 @@
 
 	default:
 		LOGP(DLLC, LOGL_NOTICE,
-		     "Unknown GSM 04.08 message type 0x%02hhx for protocol"
-		     " discriminator 0x%02hhx.\n",
-		     msg_type, pdisc);
+		     "Unhandled GSM 04.08 message type %s for protocol discriminator %s.\n",
+		     get_value_string(gprs_msgt_gmm_names, msg_type), get_value_string(gsm48_pdisc_names, pdisc));
 		break;
 	};