net: mediatek: fix mtk_pending_work

The driver supports 2 MACs. Both run on the same DMA ring. If we hit a TX
timeout we need to stop both netdevs before restarting them again. If we
don't do this, mtk_stop() wont shutdown DMA and the consecutive call to
mtk_open() wont restart DMA and enable IRQs.

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