Add defmt::Format derive to serial error

Author: Sh3Rm4n

src/serial.rs

@@ -38,6 +38,7 @@ pub enum Event {
 
 /// Serial error
 #[derive(Debug)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
 #[non_exhaustive]
 pub enum Error {
     /// Framing error