wl12xx: Read MAC address from NVS file on HW startup

Try to read the MAC address from the on-disk NVS file.
A non-zero MAC address is required to add an AP interface.

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