pciehp: add message about pciehp_slot_with_bus option

Some (broken?) platform assign the same slot name to multiple hotplug
slots. On such system, slot initialization would fail because of name
collision. The pciehp driver already have a "slot_with_bus" module
option which adds the bus number into the slot name. This patch adds
the message about this module option that will be displayed when slot
name collision is detected.

Signed-off-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Kristen Carlson Accardi <kristen.c.accardi@intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>