net: Update address for MediaTek ethernet driver in MAINTAINERS

Update maintainers for MediaTek ethernet driver with Mark Lee.
He is familiar with MediaTek mt762x series ethernet devices and
will keep following maintenance from the vendor side.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Mark Lee <Mark-MC.Lee@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>