net: phy: use of_mdio_parse_addr

use of_mdio_parse_addr() in place of an OF read of reg and a bounds
check (which is litterally the exact same thing that
of_mdio_parse_addr() does)

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