mt76: mt7615: clean up FWDL TXQ during/after firmware upload

Since we don't clean that tx queue from the tx tasklet, we need to do it
after the firmware upload is done. This patch also adds a cleanup step during
the upload, to help reclaim memory faster.

Fixes unprocessed queued frames eating up memory  long after the firmware
upload has already completed

Signed-off-by: Felix Fietkau <nbd@nbd.name>