ACPI / hotplug / PCI: Simplify hotplug_event()

A few lines of code can be cut from hotplug_event() by defining
and initializing the slot variable at the top of the function,
so do that.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Mika Westerberg <mika.westerberg@linux.intel.com>