PCI: pci_scan_slot() returns newly found devices

pci_scan_slot() has been rewritten to be less complex and will now
return the number of *new* devices found.

Existing callers need not worry because they already assume that
they can't call pci_scan_slot() on an already-scanned slot.

Thus, there is no semantic change for existing callers: returning
newly found devices (this patch) is exactly equal to returning all
found devices (before this patch).

This patch adds some more groundwork to allow us to rescan the
PCI bus during runtime to discover newly added devices.

Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Reviewed-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>