ptp: fix an IS_ERR() vs NULL check

We recently modified pps_register_source() to return error pointers
instead of NULL but it seems like there was a merge issue and part of
the commit was lost.  Anyway, the ptp_clock_register() function needs to
be updated to check for IS_ERR() as well.

Fixes: 3b1ad360ac ("pps: using ERR_PTR instead of NULL while pps_register_source fails")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/ptp/ptp_clock.c

@@ -260,7 +260,8 @@ struct ptp_clock *ptp_clock_register(struct ptp_clock_info *info,
 		pps.mode = PTP_PPS_MODE;
 		pps.owner = info->owner;
 		ptp->pps_source = pps_register_source(&pps, PTP_PPS_DEFAULTS);
-		if (!ptp->pps_source) {
+		if (IS_ERR(ptp->pps_source)) {
+			err = PTR_ERR(ptp->pps_source);
 			pr_err("failed to register pps source\n");
 			goto no_pps;
 		}