gmm: log GMM msg type name instead of number

Change-Id: I2dc6eb5bfb0f44caf2687e582d660f71fdd647a2
diff --git a/src/sgsn/gprs_gmm.c b/src/sgsn/gprs_gmm.c
index 2367338..2ba6d08 100644
--- a/src/sgsn/gprs_gmm.c
+++ b/src/sgsn/gprs_gmm.c
@@ -2111,9 +2111,9 @@
 
 null_mmctx:
 	LOGGBIUP(llme, msg, LOGL_ERROR,
-	     "Received GSM 04.08 message type 0x%02x,"
+	     "Received GSM 04.08 message type %s,"
 	     " but no MM context available\n",
-	     gh->msg_type);
+	     get_value_string(gprs_msgt_gmm_names, gh->msg_type));
 	return -EINVAL;
 }