rt2x00: Don't set unicast/BSSID masks when clearning MAC or BSSID

When configuring the MAC_ADDR or MAC_BSSID with an empty address,
the UNICAST_TO_ME_MASK and BSS_ID_MASK must also be reset to prevent
invalid interpretation of the addresses.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>