wl1271: Fix random MAC address setting

If reverting to a random MAC address, the driver would not update it to the
NVS file, and hence the firmware frame filtering would not work properly.

So update the randomized MAC address to the NVS image.

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>