pps: using ERR_PTR instead of NULL while pps_register_source fails

pps_register_source() has keeps error codes in a local variable,
but it does not make use of the code. This patch let it return
the errcode in case of failure.

Suggested-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Rodolfo Giometti <giometti@enneenne.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>