scsi: ibmvfc: Fix NULL return compiler warning

Fix up a compiler warning introduced via 54b04c99d02e

Link: https://lore.kernel.org/r/1583159961-15903-1-git-send-email-brking@linux.vnet.ibm.com
Fixes: 54b04c99d02e ("scsi: ibmvfc: Avoid loss of all paths during SVC node reboot")
Signed-off-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/ibmvscsi/ibmvfc.c

@@ -3641,7 +3641,7 @@ static void ibmvfc_tgt_implicit_logout_and_del(struct ibmvfc_target *tgt)
 	struct ibmvfc_event *evt;
 
 	if (vhost->discovery_threads >= disc_threads)
-		return NULL;
+		return;
 
 	vhost->discovery_threads++;
 	evt = __ibmvfc_tgt_get_implicit_logout_evt(tgt,