cxgb4: ptp_clock_register() returns error pointers

Check ptp_clock_register() return not only for NULL but
also for error pointers, and also nullify adapter->ptp_clock
if ptp_clock_register() fails.

Fixes: 9c33e42 ("cxgb4: Add PTP Hardware Clock (PHC) support")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/chelsio/cxgb4/cxgb4_ptp.c

@@ -441,7 +441,8 @@ void cxgb4_ptp_init(struct adapter *adapter)
 
 	adapter->ptp_clock = ptp_clock_register(&adapter->ptp_clock_info,
 						&adapter->pdev->dev);
-	if (!adapter->ptp_clock) {
+	if (IS_ERR_OR_NULL(adapter->ptp_clock)) {
+		adapter->ptp_clock = NULL;
 		dev_err(adapter->pdev_dev,
 			"PTP %s Clock registration has failed\n", __func__);
 		return;