staging: comedi: das16: fix DMA size for short transfers

Currently this driver alwasy programs the DMA transfer to be the allocated
size of the DMA buffer. When the async command 'stop_src' is TRIG_COUNT its
possible (likely) for the last transfer to be less than the buffer size.

Introduce a helper function, das16_ai_setup_dma(), to calculate the actual
size of the DMA transfer based on the number of samples remaining and the
number of unread samples.

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>