ralink: make xhci platform depend on mt7621

this broke CONFIG_ALL builds for mt7620

Signed-off-by: John Crispin <blogic@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@41471 3c298f89-4303-0410-b956-a3cf2f4a3e73