net: mediatek: use of_device_get_match_data()

The usage of of_device_get_match_data() reduce the code size a bit.

Also, the only way to call mtk_probe() is to match an entry in
of_mtk_match[], so match cannot be NULL.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>