libmsc: fix -Wenum-conversion in subscr_conn_toss()

This commit simply fixes a -Wenum-conversion thrown by clang.
No idea why are we using the SM (GPRS Session Management) cause values.
msc_a_release_mo() does not even use the given SM cause value.

Change-Id: Iade6bf97466ab2b3b39e9ea123fc90d06c0f6a9b
diff --git a/src/libmsc/sgs_iface.c b/src/libmsc/sgs_iface.c
index ad73289..a845ab8 100644
--- a/src/libmsc/sgs_iface.c
+++ b/src/libmsc/sgs_iface.c
@@ -124,7 +124,7 @@
 	LOG_MSUB(msub, LOGL_ERROR, "Force releasing previous subscriber connection: an SGs connection for this"
 		 " subscriber is being initiated\n");
 
-	msc_a_release_mo(msub_msc_a(msub), GSM48_REJECT_CONGESTION);
+	msc_a_release_mo(msub_msc_a(msub), GSM_CAUSE_AUTH_FAILED);
 	/* TODO: is this strong enough? After this, it should be completely disassociated with this subscriber. */
 }