wl12xx: enable/disable BET with AUTO_PS/ACTIVE

While the FW with dynamic PS controls BET when going to PSM and back
internally within the FW, there's still a need to enable it from the driver
at least once (so enable on every entry to AUTO_PS)
and disable it once we explicitly go back to STATION_ACTIVE_MODE.
BET isn't relevant for 5GHz

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