[media] stk-webcam: implement support for count == 0 when calling REQBUFS

The spec specifies that setting count to 0 in v4l2_requestbuffers
should result in releasing any streaming resources and the stream
ownership. Implement this.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Tested-by: Arvydas Sidorenko <asido4@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>