pxa168_eth: mdiobus_scan() doesn't return NULL anymore

Now that mdiobus_scan() doesn't return NULL on failure anymore, this driver
no  longer needs to check for it...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>