dell-laptop: fix a use-after-free error on the failure path

dell_setup_rfkill() already cleans up the rfkill devices on failure.
So if it returns an error, we should not try to unregister the rfkill
devices.

Signed-off-by: Alan Jenkins <alan-jenkins@tuffmail.co.uk>
Acked-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Len Brown <len.brown@intel.com>