ad7172: Fix RefSource From implementation

Fixes #210.

src/ad7172/mod.rs

@@ -121,9 +121,9 @@ pub enum RefSource {
 impl From<u8> for RefSource {
     fn from(x: u8) -> Self {
         match x {
-            0 => RefSource::External,
-            1 => RefSource::Internal,
-            2 => RefSource::Avdd1MinusAvss,
+            0b00 => RefSource::External,
+            0b10 => RefSource::Internal,
+            0b11 => RefSource::Avdd1MinusAvss,
             _ => RefSource::Invalid,
         }
     }