net: ethernet: mtk_wed: get rid of queue lock for tx queue

Similar to MTK Wireless Ethernet Dispatcher (WED) MCU rx queue,
we do not need to protect WED MCU tx queue with a spin lock since
the tx queue is accessed in the two following routines:
- mtk_wed_wo_queue_tx_skb():
  it is run at initialization and during mt7915 normal operation.
  Moreover MCU messages are serialized through MCU mutex.
- mtk_wed_wo_queue_tx_clean():
  it runs just at mt7915 driver module unload when no more messages
  are sent to the MCU.

Remove tx queue spinlock.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/7bd0337b2a13ab1a63673b7c03fd35206b3b284e.1673515140.git.lorenzo@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>