net: phylink: remove an_enabled

The Autoneg bit in the advertising bitmap and state->an_enabled are
always identical. state->an_enabled is now no longer used by any
drivers, so lets kill this duplication.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>