sfc: XMAC statistics fix-ups

Exclude assumed size of RX control frames from rx_bad_bytes.

Exclude assumed size of TX control frames from tx_good_bytes for
consistency with rx_good_bytes.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>