pxa168_eth: fix error handling in prope

A couple issues here:
* Some resources weren't released.
* If alloc_etherdev() failed it would have caused a NULL dereference
  because "pep" would be null when we checked "if (pep->clk)".
* Also it's better to propagate the error codes from mdiobus_register()
  instead of just returning -ENOMEM.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>