gprs_gmm: Cancel Location: use detach type IMSI

The detach type network side is defined as
- Reattach required
- Reattach not required
- IMSI detach (after VLR failure)

IMSI detach seems to be more close. Howeever the standard
isn't clear about this.

Change-Id: I27da6dc5165819cccd1ae0a98b132b45a01f38bb
diff --git a/src/gprs/gprs_gmm.c b/src/gprs/gprs_gmm.c
index 57d3883..6a5c5d9 100644
--- a/src/gprs/gprs_gmm.c
+++ b/src/gprs/gprs_gmm.c
@@ -1150,7 +1150,7 @@
 			  get_value_string(gsm48_gmm_cause_names, gmm_cause),
 			  gmm_cause);
 		gsm48_tx_gmm_detach_req(
-			ctx, GPRS_DET_T_MT_REATT_NOTREQ, gmm_cause);
+			ctx, GPRS_DET_T_MT_IMSI, gmm_cause);
 
 		mm_ctx_cleanup_free(ctx, "auth lost");
 		break;