staging: comedi: gsc_hpdi: tidy up gsc_hpdi_drain_dma()

Refactor this function to clarify the loop that reads all the full
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>