pciehp: Event handling rework

The event handler of PCIEHP driver is unnecessarily very complex. In
addition, current event handler can only a fixed number of events at
the same time, and some of events would be lost if several number of
events happened at the same time.

This patch simplify the event handler using 'work queue', and it also
fix the above-mentioned issue.

Signed-off-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Kristen Carlson Accardi <kristen.c.accardi@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>