Commit b03e7495 ("PCI: Set PCI-E Max Payload Size on fabric")
introduced a potential NULL pointer dereference in calls to
pcie_bus_configure_settings due to attempts to access pci_bus self
variables when the self pointer is NULL.

To correct this, verify that the self pointer in pci_bus is non-NULL
before dereferencing it.
Reported-by: NStanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: NShyam Iyer <shyam_iyer@dell.com>
Signed-off-by: NJon Mason <mason@myri.com>
Acked-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>