net: of_mdio: use int type for address variable

Use int rather than u32 to fix the following warning:

drivers/of/of_mdio.c:147 of_mdiobus_register() warn: unsigned 'addr' is
never less than zero.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Fixes: 8f838288 ("net: of_mdio: factor out code to parse a phy's 'reg' property")
Signed-off-by: David S. Miller <davem@davemloft.net>