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: 3b1ad360 ("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>