mt76: mt76x02u: move mt76x02u_mac_start in mt76x02-usb module

Unify mt76x02u_mac_start between mt76x2u and mt76x0u since the
code is shared between both drivers

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>