ath9k: calcrxfilter should take multiple VIFs into account.

When there is more than one VIF, listen for all beacons
and ensure ATH9K_RX_FILTER_MCAST_BCAST_ALL is set.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>