sfc: Add channel specific receive_skb handler and post_remove callback

Allows an extra channel to override the standard receive_skb handler
and also for extra non generic operations to be performed on remove.

Also set default rx strategy so only skbs can be delivered to the
PTP receive function.

Signed-off-by: Stuart Hodgson <smhodgson@solarflare.com>
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>