net: mdio-uclass: rename arguments of dm_mdio_phy_connect for clarity

Renamed dm_mdio_phy_connect arguments dev to mdiodev and addr to phyaddr
for a bit more clarity and consistency with the following patches.
Also use NULL instead of 0 on error return path.

Signed-off-by: Alex Marginean <alexandru.marginean@nxp.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>