net: ethernet: ti: davinci_cpdma: free memory while channel destroy

While create/destroy channel operation memory is not freed. It was
supposed that memory is freed while driver remove. But a channel
can be created and destroyed many times while changing number of
channels with ethtool.

Based on net-next/master

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>