net: phy: convert read-modify-write to phy_modify()

Convert read-modify-write sequences in at803x, Marvell and core phylib
to use phy_modify() to ensure safety.

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