sfc: split setup of hardware timestamping into NIC-type operation

I added efx_ptp_get_mode() to avoid moving the definition for
efx_ptp_data, since the current PTP mode is needed for
siena.c:siena_set_ptp_hwtstamp.

[bwh: Also move the rx_filters mask, and add kernel-doc]
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>