media: usb: usbtv: Stop direct calls to queue num_buffers field

Use vb2_get_num_buffers() to avoid using queue num_buffers field directly.
This allows us to change how the number of buffers is computed in the
future.

Signed-off-by: Benjamin Gaignard <[e-mail]>
Reviewed-by: Andrzej Pietrasiewicz <[e-mail]>
Signed-off-by: Hans Verkuil <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

drivers/media/usb/usbtv/usbtv-video.c

···
 {
 	struct usbtv *usbtv = vb2_get_drv_priv(vq);
 	unsigned size = USBTV_CHUNK * usbtv->n_chunks * 2 * sizeof(u32);
+	unsigned int q_num_bufs = vb2_get_num_buffers(vq);
 
-	if (vq->num_buffers + *nbuffers < 2)
-		*nbuffers = 2 - vq->num_buffers;
+	if (q_num_bufs + *nbuffers < 2)
+		*nbuffers = 2 - q_num_bufs;
 	if (*nplanes)
 		return sizes[0] < size ? -EINVAL : 0;
 	*nplanes = 1;