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>