staging: comedi: pcl812: fix logic error in pcl812_ai_setup_dma()

commit 92afc2b2 inroduced a logic error
in the DMA size calculation.

If the 'nsamples' is greater than the 'unread_samples' then DMA needs to
be restarted. The current code checks it agains the 'max_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>