firewire: cdev: shut down iso context before freeing the buffer

DMA must be halted before we DMA-unmap and free the DMA buffer.  Since
we cannot rely on the client to stop the context before it closes the
fd, we have to reorder fw_iso_buffer_destroy vs. fw_iso_context_destroy.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>