ath9k: Configure RX filter for multi-BSSID broadcast

Allow RX filter to pass through all broadcast/multicast frames (i.e.,
no BSSID filtering) if virtual interfaces are used. Software filtering
will be used in this case to drop broadcast/multicast frames for
foreign BSSIDs.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>