net: ethernet: ti: cpsw: use destroy ctlr to destroy channels

There is no reason to destroy channels that are destroyed while
cpdma_ctlr destroy. In this case no need to remember how much
channels where created and destroy them by one, as cpdma_ctlr
destroys all of them.

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>