intel_th: Make SOURCE devices children of the root device

The switch (GTH) does not directly interact with SOURCE type devices and
may not even be present (in host mode). To reflect this and avoid
inconsistencies between target and host mode, make SOURCE devices
descendant directly from the root (i.e. PCI) device. Their symlinks
will no longer appear under the switch device, but they can still
be found under intel_th bus.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>