wl1271: Retrieve device mac address from the nvs file

Instead of always randomizing the MAC address, retrieve and configure the
MAC address from the device specific nvs file. For now, randomize an address
only if the address in the NVS is zero.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>