pciehp: Fix wrong slot control register access

Current pciehp implementaion clears hotplug events without waiting for
command completion. Because of this, events might not be cleared properly.
To prevent this problem, we must use pciehp_write_cmd() to write to
Slot Control register.

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>