powerpc/eeh: Fix fetching bus for single-dev-PE

commit ea461abf upstream.

While running Linux as guest on top of phyp, we possiblly have
PE that includes single PCI device. However, we didn't return
its PCI bus correctly and it leads to failure on recovery from
EEH errors for single-dev-PE. The patch fixes the issue.

Signed-off-by: Gavin Shan <shangw@linux.vnet.ibm.com>
Cc: Steve Best <sbest@us.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>