staging: comedi: das1800: introduce das1800_free_dma()

For aesthetics, introduce a helper function to free the DMA channels and
allocated buffers.

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>