net-next: mediatek: split IRQ register locking into TX and RX

Originally the driver only utilised the new QDMA engine. The current code
still assumes this is the case when locking the IRQ mask register. Since
RX now runs on the old style PDMA engine we can add a second lock. This
patch reduces the IRQ latency as the TX and RX path no longer need to wait
on each other under heavy load.

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: David S. Miller <davem@davemloft.net>