ath79: ag71xx: update ethtool support

ath79: ag71xx: update ethtool support

ethtool doesn't work currently as phy_ethtool_ioctl expects user space
pointer, but it's being passed kernel one. Fixing it doesn't make sense
as {s,g}et_settings were deprecated anyway.  So let's rather remove
phy_ethtool_ioctl and use new {s,g}et_link_ksettings instead. While at
it, update nway_reset as well.

Cc: John Crispin <john@phrozen.org>
Ref: https://bugs.openwrt.org/index.php?do=details&task_id=1982
Signed-off-by: Petr Štetiar <ynezz@true.cz>