mt76: testmode: add support to set MAC

Add support to set SA/DA/BSSID in testmode.
During mass production stage, some test scripts or test equipments need
to set fixed addresses to detect or parse if a test is passed.
Also, MAC setting is necessary for some tx/rx tests with rx filter,
to make sure rx site only receives expected packets.

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