mt76: discard early received packets if not running yet

If the radio was previously in running state, it can receive some
packets before it is able to process them.
This can lead to a crash if the channel is not initialized yet.
Discard all rx packets until start() is called

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>