[media] mem2mem_testdev: return pending buffers in stop_streaming()

To keep the vb2 buffer administration in balance stop_streaming() must
return any pending buffers to the vb2 framework.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>