MIPS: ath79: Don't return PCIBIOS_* code from pcibios_enable_device()

pcibios_plat_dev_init() is called from pcibios_enable_device() that
should return normal errnos, not PCIBIOS return codes. In this case the
impact is only cosmetic because PCIBIOS_SUCCESSFUL equals 0 that is
success code with errnos as well. Nonetheless, remove the inconsistency
by replacing the PCIBIOS_SUCCESSFUL with 0.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Sergio Paracuellos <sergio.paracuellos@gmal.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>