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.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

MAINTAINERS

@@ -11757,6 +11757,7 @@ F:	drivers/char/hw_random/mtk-rng.c
 MEDIATEK SWITCH DRIVER
 M:	Sean Wang <sean.wang@mediatek.com>
 M:	Landen Chao <Landen.Chao@mediatek.com>
+M:	DENG Qingfang <dqfext@gmail.com>
 L:	netdev@vger.kernel.org
 S:	Maintained
 F:	drivers/net/dsa/mt7530.*