net: ethernet: ti: cpsw_ethtool: fix ethtool ring param set

Fix ability to set RX descriptor number, the reason - initially
"tx_max_pending" was set incorrectly, but the issue appears after
adding sanity check, so fix is for "sanity" patch.

Fixes: 37e2d99b

 ("ethtool: Ensure new ring parameters are within bounds during SRINGPARAM")
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>