[media] media: blackfin: bfin_capture: set min_buffers_needed

this patch sets the min_buffers_needed field of the vb2 queue
so that the vb2 core will make sure start_streaming() callback
is called only when we have minimum buffers queued.

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Tested-by: Scott Jiang <scott.jiang.linux@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>