mdio: Demote print from info to debug in mdio_device_register

While it is useful to know which MDIO device is being registered, demote
the dev_info() to a dev_dbg().

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>