staging: comedi: quatech_daqp_cs: cleanup Step 3 of ai (*do_cmdtest)

Step 3 of the (*do_cmdtest) trivially validates the async command arguments.
The validations also modify the arguments if they are invalid so that the
user gets valid values if the test fails.

Reorder the checks so that if any of the checks fail proper values are
used for subsequent checks.

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>