phylib: use MDIO_DEVS[12]

The bare register numbers are used despite <uapi/linux/mdio.h> has MDIO_DEVS[12]
#define'd for those.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>