staging: comedi: drivers: introduce comedi_nsamples_left()

Introduce a helper function to calculate the number of samples remaining
when the cmd->stop_src is TRIG_COUNT.

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>