mt76: mt76x0: eeprom: add support for MAC address from OF

mt76x0e driver only supports MAC addresses from
calibration data eeprom. Many routers however do not
have a valid stock address set in this field.

This patch makes it possible to take a MAC address
from OF (e.g. from mtd).

Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
[adjusted for kernel submission]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Signed-off-by: Felix Fietkau <nbd@nbd.name>