sfc: Add support for TX MAC filters

On Siena each TX queue can be configured to send only packets for
which there is a TX MAC filter that matches the source MAC address,
queue ID, and optionally VID.  This will be used to implement the
'spoofchk' feature for SR-IOV virtual functions.

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