pounder: fix attenuator indices (latch and shiftreg)

Before the discriminant was used despite being a compiler implementaiton
detail. This now fixes the discriminant to match byte index in the attenuator
shift register and latch-enable index of the gpio extender.

src/hardware/pounder/attenuators.rs

@@ -35,7 +35,7 @@ pub trait AttenuatorInterface {
         // The lowest 2 bits of the 8-bit shift register on the attenuator are ignored. Shift the
         // attenuator code into the upper 6 bits of the register value. Note that the attenuator
         // treats inputs as active-low, so the code is inverted before writing.
-        channels[channel as usize] = (!attenuation_code) << 2;
+        channels[channel as usize] = !(attenuation_code << 2);
         self.transfer_attenuators(&mut channels)?;
 
         // Finally, latch the output of the updated channel to force it into an active state.

src/hardware/pounder/mod.rs

@@ -41,10 +41,10 @@ pub enum Error {
 #[derive(Debug, Copy, Clone)]
 #[allow(dead_code)]
 pub enum Channel {
-    In0,
-    In1,
-    Out0,
-    Out1,
+    In0 = 0,
+    Out0 = 1,
+    In1 = 2,
+    Out1 = 3,
 }
 
 #[derive(Serialize, Deserialize, Copy, Clone, Debug)]
@@ -340,12 +340,7 @@ impl AttenuatorInterface for PounderDevices {
     /// Args:
     /// * `channel` - The attenuator channel to latch.
     fn latch_attenuator(&mut self, channel: Channel) -> Result<(), Error> {
-        let pin = match channel {
-            Channel::In0 => 0,
-            Channel::Out0 => 1,
-            Channel::In1 => 2,
-            Channel::Out1 => 3,
-        };
+        let pin = channel as u8;
         self.mcp23017
             .write_gpio(mcp23017::Port::GPIOB, 0x2f & !(1 << pin))
             .map_err(|_| Error::I2c)?;