gprs: Fix GSUP cancel_type handling (Coverity)

When handling an incoming GSUP cancellation request, the cancel_type
if effectively ignored, such that is always handled as
GPRS_GSUP_CANCEL_TYPE_UPDATE and never as WITHDRAW.

This commit fixes the expression used to set the variable
is_update_procedure.

Fixes: Coverity CID 1267739
Sponsored-by: On-Waves ehf
diff --git a/openbsc/src/gprs/gprs_subscriber.c b/openbsc/src/gprs/gprs_subscriber.c
index 8486834..e3da0f8 100644
--- a/openbsc/src/gprs/gprs_subscriber.c
+++ b/openbsc/src/gprs/gprs_subscriber.c
@@ -492,7 +492,8 @@
 					     struct gprs_gsup_message *gsup_msg)
 {
 	struct gprs_gsup_message gsup_reply = {0};
-	int is_update_procedure = !gsup_msg->cancel_type || gsup_msg->cancel_type;
+	int is_update_procedure = !gsup_msg->cancel_type ||
+		gsup_msg->cancel_type == GPRS_GSUP_CANCEL_TYPE_UPDATE;
 
 	LOGGSUBSCRP(LOGL_INFO, subscr, "Cancelling MS subscriber (%s)\n",
 		    is_update_procedure ?