wl18xx: derive the MAC address from the BD_ADDR in fuse ROM

Add the get_mac operation in order to fetch the BD_ADDR from fuse ROM,
so that we can derive the WLAN MAC addresses from it.

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