mt76x0: use bus helper to identify rf access method

Use mt76_is_usb() to identify RF access method instead of
MT76_STATE_MCU_RUNNING flag and add warning since MCU has
to be initialized before we can access RF registers via MCU.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>