net: phy: mscc: add constants for used interrupt mask bits

Add constants for the used interrupts bits. This avoids the magic
number for MII_VSC85XX_INT_MASK_MASK.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>