net: ethernet: davinci_cpdma: decrease the desc count when cleaning up the remaining packets

chan->count is used by rx channel. If the desc count is not updated by
the clean up loop in cpdma_chan_stop, the value written to the rxfree
register in cpdma_chan_start will be incorrect.

Signed-off-by: Tao Hou <hotforest@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>