rt2x00: add support for RT5350 WiSoC

Support for the RT5350 WiSoC was added to OpenWrt after having a
lengthy debate about the legality of the original submission, see
https://lists.openwrt.org/pipermail/openwrt-devel/2013-January/018224.html
MTK/Ralink Acked replied and says we can merge this patch under the GPL.
https://dev.openwrt.org/changeset/36177



Signed-off-by: Serge Vasilugin <vasilugin@yandex.ru>
Tested-by: Michel Stempin <michel.stempin@wanadoo.fr>
Acked-by: John Crispin <blogic@openwrt.org>
[daniel@makrotopia.org: added commit message, cleaned up code]
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>