hwtracing: intel_th: use dev_groups and not dev_attrs for bus_type

The dev_attrs field has long been "depreciated" and is finally being
removed, and as this driver isn't even using it, just drop the NULL
setting, it is pointless.

Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>