phy: micrel: Use MICREL_PHY_ID_MASK definition

Replace the hardcoded mask 0x00fffff0 with MICREL_PHY_ID_MASK for
better readability.

Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>