net: phylink: use mdiobus_modify_changed() helper

Use the mdiobus_modify_changed() helper in the C22 PCS advertisement
helper.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>