cns3xxx: ethernet - clean the tx ring only in the poll function, not in the xmit handler

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@35952 3c298f89-4303-0410-b956-a3cf2f4a3e73
cns3xxx: ethernet - clean the tx ring before refilling the rx ring to reduce memory pressure

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@35953 3c298f89-4303-0410-b956-a3cf2f4a3e73