mt76: fix possible undetected invalid MAC address

Make sure the MAC address is checked before function returns.

If CONFIG_OF is set and the device node is null, the function will
return directly, and an invalid MAC address will not be checked.

Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>