net: phylink: simplify ksettings_set() implementation

Simplify the ksettings_set() implementation to look more like phylib's
implementation; use a switch() for validating the autoneg setting, and
use the linkmode_modify() helper to set the autoneg bit in the
advertisement mask.

Reviewed-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>