Fix mt7615_init_tx_queues() so that it returns result value of final call
to mt7615_init_tx_queue() instead of default 0. Otherwise, failure in
mt7615_init_tx_queue() is not tracked by the parent function. The patch
can be cleanly applied to the 5.10 branch.

This issue is addressed in b671da33d1c5 upstream. I decided to refrain
from backporting it whole for now.