wl12xx: always initialize AP-mode max power level

Sometimes we only get the Tx power level via op->config when the FW is
off. Record the received power level when this happens and use it to
initialize the firmware during boot.

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