adc: demo_mode: fix divide by zero for channels with parents

The simulated total USB Host current is calculates by summing up the
individual host currents. This channel does not have anything like
exponential decay and thus sets the time constants as zero.
Prevent divide by zero and provide immediate decay.

Signed-off-by: Leonard Göhrs <[email protected]>

Author: hnez

src/adc/iio/demo_mode.rs

@@ -128,8 +128,14 @@ impl CalibratedChannel {
 
         let mut value = f32::from_bits(self.inner.value.load(Ordering::Relaxed));
 
+        let decay = if time_constant.abs() < 0.01 {
+            0.0
+        } else {
+            (-dt / time_constant).exp()
+        };
+
         value -= nominal;
-        value *= (-dt / time_constant).exp();
+        value *= decay;
         value += (2.0 * thread_rng().gen::<f32>() - 1.0) * self.inner.noise;
         value += self
             .inner