staging: comedi: me4000: validate the cmd->start_arg

The cmd->start_arg should be trivially validated in Step 3 of the
(*do_cmdtest) functions. Add the necessary code.

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>