net: phy: marvell: fix typo in constant MII_M1011_PHY_SRC_DOWNSHIFT_MASK

Fix typo and use PHY_SCR for PHY-specific Control Register.

Fixes: a3bdfce7

 ("net: phy: marvell: support downshift as PHY tunable")
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>