net: realtek: r8169: implement set_link_ksettings()

Commit 6fa1ba61 partially
implemented the new ethtool API, by replacing get_settings()
with get_link_ksettings(). This breaks ethtool, since the
userspace tool (according to the new API specs) never tries
the legacy set() call, when the new get() call succeeds.

All attempts to chance some setting from userspace result in:
> Cannot set new settings: Operation not supported

Implement the missing set() call.

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>