net: Update MAINTAINERS for MediaTek switch driver

Update maintainers for MediaTek switch driver with Landen Chao who is
familiar with MediaTek MT753x switch devices and will help maintenance
from the vendor side.

Cc: Steven Liu <steven.liu@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Landen Chao <Landen.Chao@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>