phy: make phy_connect_fixed work with a null mdio bus

It is utterly pointless to require an MDIO bus pointer for a fixed PHY
device. The fixed.c implementation does not require it, only
phy_device_create. Fix that.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>