ag71xx: fix memory corruption issues on ar7240 on ethernet start/stop

ag71xx: fix memory corruption issues on ar7240 on ethernet start/stop

When the DMA engine state gets corrupted due to a hardware issues, it
often won't stop rx until a full reset is issued. In that case the hardware
must keep a valid descriptor, otherwise it will write to random places in
system RAM, triggering random crashes. To fix this, keep a dummy descriptor
without a buffer that keeps the DMA engine in a sane state until the reset
is done

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@27895 3c298f89-4303-0410-b956-a3cf2f4a3e73