cns3xxx: minor ethernet driver cleanup

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@35949 3c298f89-4303-0410-b956-a3cf2f4a3e73
cns3xxx: prevent the ethernet driver from accessing uninitialized dma descriptor entries

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@35950 3c298f89-4303-0410-b956-a3cf2f4a3e73
cns3xxx: increase number of ethernet driver rx buffers

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@35951 3c298f89-4303-0410-b956-a3cf2f4a3e73