wl1251: Generate random MAC address only if driver does not have valid

Before this patch, driver generated random MAC address every time it was
initialized. After that random MAC address could be overwritten with fixed
one, if provided.

This patch changes order. First it tries to read fixed MAC address and if
it fails then driver generates random MAC address.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>