powerpc/82xx: kmalloc failure ignored in ep8248e_mdio_probe()

Prevent NULL dereference if kmalloc() fails. Also clean up if
of_mdiobus_register() returns an error.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Acked-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>