net: phy: remove phy_ethtool_sset()

There are no users of phy_ethtool_sset() in the kernel anymore, and
as of commit 3c1bcc86

 ("net: ethernet: Convert phydev advertize
and supported from u32 to link mode"), the implementation is slightly
buggy - it doesn't correctly check the masked advertising mask as it
used to.

Remove it, and update the phy documentation to refer to its replacement
function.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>