net: mediatek: move the pending_work struct to the device generic struct

The worker always touches both netdevs. It is ethernet core and not MAC
specific. We only need one worker, which belongs into the ethernets core
struct.

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