net/sfc: provide a way to check if filter is supported

The information is obtained from firmware on attach. It may
change after MC reboot (firmware version or variant change).
Cache should be refreshed after MC reboot when it is handled
properly (not yet).

Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>
Reviewed-by: Andrew Lee <alee@solarflare.com>
Reviewed-by: Andy Moreton <amoreton@solarflare.com>
Reviewed-by: Robert Stonehouse <rstonehouse@solarflare.com>