wlcore: don't enable BET for high basic rates

The beacon early termination is not relevant for
high basic rates, which doesn't contribute
anything to the PS and only adds unnecessary FW
work.

Enable BET only if the basic rate is less than 9
Mbps.

Signed-off-by: Ziv Riesel <zivriesel@ti.com>
Signed-off-by: Victor Goldenshtein <victorg@ti.com>
Signed-off-by: Igal Chernobelsky <igalc@ti.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>