thunderbolt: Use kobj_to_dev() instead of container_of()

Doesn't really matter for an individual driver, but it may
get coppied to lots more. I consider it's a little tidy up.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>