wl12xx: export driver state to debugfs

By reading the "driver_state" debugfs value we get all the important
state information from the wl12xx driver. This helps testing and
debugging, particularly in situations where the driver seems "stuck".

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