net: Update MAINTAINERS for MediaTek switch driver

Update maintainers for MediaTek switch driver with Deng Qingfang who has
contributed many high-quality patches (interrupt, VLAN, GPIO, and etc.)
and will help maintenance.

Signed-off-by: Landen Chao <landen.chao@mediatek.com>
Signed-off-by: DENG Qingfang <dqfext@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/49e1aa8aac58dcbf1b5e036d09b3fa3bbb1d94d0.1626751861.git.landen.chao@mediatek.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>