if there is any new tx need to clean up. do it in next napi poll.
collect tx related members to fe_tx_ring struct. for better
cache usage and more readable.
Signed-off-by: michael lee <igvtee@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45895 3c298f89-4303-0410-b956-a3cf2f4a3e73