ACPI / hotplug / PCI: Always return success after adding a function

When a new ACPIPHP function is added by register_slot() and the
notify handler cannot be installed for it, register_slot() returns an
error status without cleaning up, which causes the entire namespace
walk in acpiphp_enumerate_slots() to be aborted, although it still
may be possible to successfully install the function notify handler
for other device objects under the given brigde.

To address this issue make register_slot() return success after
a new function has been added, even if the addition of the notify
handler for it has failed.

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