staging: vc04_services: codec: Fix incorrect buffer cleanup

The allocated input and output buffers are initialised in
buf_init and should only be cleared up in buf_cleanup.
stop_streaming was (incorrectly) cleaning up the buffers to
avoid an issue in videobuf2 that had been fixed by the orphaned
buffer support.

Remove the erroneous cleanup.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>