stm class: Fix device leak in open error path

Make sure to drop the reference taken by class_find_device() also on
allocation errors in open().

Signed-off-by: Johan Hovold <johan@kernel.org>
Fixes: 7bd1d409

 ("stm class: Introduce an abstraction for...")
Cc: stable@vger.kernel.org # v4.6+
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>