wl12xx: keep beacon-filtering enabled during STA operation

Enable beacon filtering on STA init, and don't disable it when entering
active mode. Otherwise dynamic-PS supports means we receive beacons from
the current AP during any Tx/Rx performed by the driver.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>