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>