net: netcp: fix check of requested timestamping filter

The driver doesn't support timestamping of all received packets and
should return error when trying to enable the HWTSTAMP_FILTER_ALL
filter.

Cc: WingMan Kwok <w-kwok2@ti.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>