sfc: support second + quarter ns time format for receive datapath

The time_format that we stash in the PTP data structure is never
 referenced, so we can remove it.  Instead, store the information needed
 to interpret sync event timestamps.
Also rolls in a couple of other related minor PTP fixes.

Based on patches by Bert Kenward <bkenward@solarflare.com> and Laurence
 Evans <levans@solarflare.com>.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>