ACPI / PCI / hotplug: unlock in error path in acpiphp_enable_slot()

In acpiphp_enable_slot(), there is a missing unlock path
when error occurred.  It needs to be unlocked before returning
an error.

Signed-off-by: Insu Yun <wuninsu@gmail.com>
Cc: All applicable <stable@vger.kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>