media: stk-webcam: Fix use after free on disconnect

We free the stk_camera device too early.  It's allocate first in probe
and it should be freed last in stk_camera_disconnect().

Reported-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>