wl12xx: add beacon_filtering debugfs file

Allow enabling/disabling beacon_filtering by debugfs,
in order to reduce the log size while debugging (beacon
filtering is disabled by default in AP mode, as beacons
are needed for ERP configuration).

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>