net: ethernet: ti: cpsw: clarify ethtool ops changing num of descs

After adding cpsw_set_ringparam ethtool op, better to carry out
common parts of similar ops splitting descriptors in runtime. It
allows to reuse these parts and shows what the ops actually do.

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