net: phylink: simplify fixed-link case for ksettings_set method

For fixed links, we only allow the current settings, so this should be
a matter of merely rejecting an attempt to change the settings.  If the
settings agree, then there is nothing more we need to do.

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>