net-next: mediatek: fix compile error inside mtk_poll_controller()

commit 80673029

 ("net-next: mediatek: add support for IRQ grouping")
failed to properly update the irq handling inside mtk_poll_controller()
causing compile errors if netconsole was enabled. Fix this by updating
the code to use the new separated irq handler function for RX.

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