ralink: fix tx vlan offload and hardware status

hardware status and tx vlan offload support on all targets
except rt5350. so i modify the IS_ENABLE condition only for
mt7621.
support mt7621 hardware status reference by SDK. but i don't
have mt7621. if not work just set mt7621 FE_REG_FE_COUNTER_BASE
to 0 to let software count.

Signed-off-by: michael lee <igvtee@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43303 3c298f89-4303-0410-b956-a3cf2f4a3e73