pciehp: poll cmd completion if hotplug interrupt is disabled

Fix improper long wait for command completion in pciehp probing.

As described in PCI Express specification, software notification is
not generated if the command that occurs as a result of a write to the
Slot Control register that disables software notification of command
completed events. Since pciehp driver doesn't take it into account,
such command is issued in pciehp probing, and it causes improper long
wait for command completion.

This patch changes the pciehp driver to take such command into
account.

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>