shpchp: check firmware before taking control

Fix the following problems of shpchp driver about getting hotplug
control from firmware.

  - The shpchp driver must not control the hotplug controller if it
    fails to get control from the firmware. But current shpchp
    controls the hotplug controller regardless the result, because it
    doesn't check the return value of get_hp_hw_control_from_firmware().

  - Current shpchp driver doesn't support _OSC.

The pciehp driver already have the code for evaluating _OSC and OSHP
and shpchp and pciehp can share it. So this patch move that code from
pciehp to acpi_pcihp.c.

Signed-off-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>