net: phy: implement fallback mechanism for negative phy adresses

Negative phy-addresses can occour if the caller function was not able to
determine a valid phy address (from device-tree for example). In this
case we catch this here and search for ANY phy device on the given mdio-
bus.

Signed-off-by: Hannes Schmelzer <hannes.schmelzer@br-automation.com>
Tested-by: Michal Simek <michal.simek@xilinx.com>
Tested-by: Lukasz Majewski <lukma@denx.de>