net: ethernet: ti: cpsw_ethtool: simplify slave loops

Only for consistency reasons, do it like in main cpsw.c module
and use ndev reference but not by means of slave.

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