staging: comedi: usbduxfast: tidy up the conversion timing validation

Cleanup the validation of the cmd->convert_arg to use the same calculation
in the (*do_cmdtest) and the (*do_cmd).

Remove the, then unnecessary, redundant check in the (*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>