net: ethernet: ti: davinci_cpdma: add set rate for a channel

The cpdma has 8 rate limited tx channels. This patch adds
ability for cpdma driver to use 8 tx h/w shapers. If at least one
channel is not rate limited then it must have higher number, this
is because the rate limited channels have to have higher priority
then not rate limited channels. The channel priority is set in low-hi
direction already, so that when a new channel is added with ethtool
and it doesn't have rate yet, it cannot affect on rate limited
channels. It can be useful for TSN streams and just in cases when
h/w rate limited channels are needed.

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