staging: comedi: dmm32at: tidy up cmd->convert_{src, arg} validation

This driver only supports a single convert_src, TRIG_TIMER. Tidy up
the (*do_cmdtest) validation of the cmd->convert_{src,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>