net: phy: marvell10g: fix usage of new MMD modifying helpers

When replacing mv3310_modify() with phy_modify_mmd() we missed that
they behave differently, mv3310_modify() returns 1 on a changed
register value whilst phy_modify_mmd() returns 0. Fix this by replacing
phy_modify_mmd() with phy_modify_mmd_changed() where needed.

Fixes: b52c018d ("net: phy: make use of new MMD accessors")
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>