net: ethernet: ti: cpsw: extend limits for cpsw_get/set_ringparam

Allow to set number of descs close to possible values. In case of
minimum limit it's equal to number of channels to be able to set
at least one desc per channel. For maximum limit leave enough descs
number for tx channels.

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