staging: comedi: me4000: validate cmd->stop_arg in step 3 of (*do_cmdtest)

The async command arguments are supposed to be trivially validated in step 3
of the (*do_cmdtest). Fix the validation of the stop_arg.

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>