mt76: mt76s: fix NULL pointer dereference in mt76s_process_tx_queue

Fix a possible NULL pointer dereference in mt76s_process_tx_queue that
can occur if status thread runs before allocating tx queues

Fixes: 6a618acb ("mt76: sdio: convert {status/net}_work to mt76_worker")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/b49c1b4edacd87b2241a9fd0431dd4864c8963f6.1607418933.git.lorenzo@kernel.org