sfc: Stop/re-start PTP when stopping/starting the datapath.

This disables PTP when we bring the interface down to avoid getting
unmatched RX timestamp events, and tries to re-enable it when bringing
the interface up.

[bwh: Make efx_ptp_stop() safe on Falcon. Introduce
 efx_ptp_{start,stop}_datapath() functions; we'll expand them later.]

Fixes: 7c236c43

 ('sfc: Add support for IEEE-1588 PTP')
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>