ath9k: Set BSSID mask based on configured interfaces

Instead of using a hardcoded BSSID mask (mask for own addresses),
iterate through all active interfaces and determine the minimal mask
that covers all local addresses.

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