PCI: Make pci_scan_slot more robust

Yinghai pointed out that the new pci_scan_slot() crashes when called
on an ARI-capable slot that is empty.  Fix this by exiting early from
pci_scan_slot if there is no device in the slot.

Also make next_ari_func() robust against devices not existing in case
the ARI capability is corrupt.  ARI also requires that the devices be
listed in order, so if we find a function listed that is out of order,
stop scanning to prevent loops.

Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>