mdio: Demote print from info to debug in mdio_driver_register

While it is useful to know which MDIO driver is being registered, demote
the pr_info() to a pr_debug().

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>