MAINTAINERS: update MEDIATEK ETHERNET entry

Update MEDIATEK ETHERNET driver maintainer file enty adding myself to
maintainers list

Acked-by: Felix Fietkau <nbd@nbd.name>
Acked-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>