PCI: Fix faulty logic in pci_reset_bus()

The pci_reset_bus() function calls pci_probe_reset_slot() to determine
whether to call the slot or bus reset.  The check has faulty logic in that
it does not account for pci_probe_reset_slot() being able to return an
errno.  Fix by only calling the slot reset when the function returns 0.

Fixes: 811c5cb3 ("PCI: Unify try slot and bus reset API")
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Cc: Sinan Kaya <okaya@codeaurora.org>