[media] vb2: fix num_buffers calculation if req->count > VIDEO_MAX_FRAMES

num_buffers can't be bigger than VIDEO_MAX_FRAME. This is assured by:
  num_buffers = min_t(unsigned int, req->count, VIDEO_MAX_FRAME);

However, this value is overriden by:
  num_buffers = max_t(unsigned int, req->count, q->min_buffers_needed);

It should, instead, use the previously calculated value as an input
to max_t:
  num_buffers = max_t(unsigned int, num_buffers, q->min_buffers_needed);

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>