staging: comedi: das800: remove extra divisor calculation call

The comedi core always calls the (*do_cmdtest) function before it
calls the (*do_cmd) function. The divisors were already calculated
in das800_ai_do_cmdtest(), it is not necessary to calculate them
again in das800_ai_do_cmd().

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>