net: phylink: use config.an_enabled in ksettings_set method

Rather than recomputing whether AN is enabled, use config.an_enabled.

Suggested-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>