ACPI / hotplug / PCI: Do not check SLOT_ENABLED in enable_device()

With Thunderbolt you can daisy chain devices: connect new devices to
an already plugged one.  In that case the "hotplug slot" is already
enabled, but we still want to look for new PCI devices behind it.

Reuse enable_device() to scan for new PCI devices on enabled slots
and push the SLOT_ENABLED check up into acpiphp_enable_slot().

[rjw: Rebased, modified the changelog]
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>