staging: comedi: quatech_daqp_cs: avoid calculating pacer divisor multiple times

The pacer will either be used to trigger each conversion or to start each
scan. Modify the (*do_cmdtest) so that one one divisor calculation is needed.
Save the divisor value in the private data so that the (*do_cmd) does not
have to do the calculation again.

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>