ralink: add support for mt7621 switch counters

Signed-off-by: Cristian Morales Vega <cristian@samknows.com>

Backport of r46050

git-svn-id: svn://svn.openwrt.org/openwrt/branches/chaos_calmer@46060 3c298f89-4303-0410-b956-a3cf2f4a3e73