net: phy: marvell: use phy_modify_changed()

Use phy_modify_changed() to change the fiber advertisement register
rather than open coding this functionality.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>