net: phy: mdio-bcm-iproc: Do not show kernel pointer

Displaying the virtual address at which the MDIO base register address
has been mapped is not useful and is not visible with pointer hashing in
place, replace the message with something indicating successful
registration instead.

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>