mdio_bus: get_phy_device() doesn't return NULL anymore

Now that get_phy_device() no longer returns NULL on error, we don't need
to check for it...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>