wl1251: read default MAC address from EEPROM when available

Some wl1251 hardware configurations (like in WG7210 module) have
EEPROM attached where NVS data is kept, which includes MAC address.

In such configurations, let's read default MAC address from EEPROM,
instead of using random one.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Acked-by: Kalle Valo <kvalo@adurom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>