mt76: use mt76_dev in mt76_is_{mmio,usb}

Convert mt76_is_mmio and mt76_is_usb to rely on mt76_dev instead of
mt76x02_dev since this is a property not strictly related to hw chipset
and it will be more reusable

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