sh: sh_eth: Add support ethtool

This commit supports following functions.
  - get_settings
  - set_settings
  - nway_reset
  - get_msglevel
  - set_msglevel
  - get_link
  - get_strings
  - get_ethtool_stats
  - get_sset_count

About other function, the device does not support.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>