staging: comedi: ni_labpc_isadma: tidy up labpc_drain_dma()

Tidy up the code that determines the number of samples to read for the
current DMA transfer and how many samples are needed for the next DMA,

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>