staging: comedi: das16: introduce das16_free_dma()

For aesthetics, move the freeing of the DMA channel and the buffers to
a helper function.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>