wl12xx: Enable beacon early termination in 2.4GHz band only

Beacon early termination doesn't help much in the 5GHz band and masks
channel switch IE Beacons.  Thus, change the code to use BET only in
2.4GHz.

[Reworded the commit log slightly -- Luca.]

Signed-off-by: Shahar Levi <shahar_levi@ti.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>