mt7601u: check memory allocation failure

Check memory allocation failure and return -ENOMEM in such a case, as
already done a few lines below.

As 'dev->tx_q' can be NULL, we also need to check for that in
'mt7601u_free_tx()', and return early.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Jakub Kicinski <kubakici@wp.pl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>