Fix clippy warnings

Author: nisembedded

src/serial/usart.rs

@@ -184,7 +184,7 @@ where
     Serial<USART>: hal::serial::Write<u8>,
 {
     fn write_str(&mut self, s: &str) -> fmt::Result {
-        let _ = s.as_bytes().iter().map(|c| block!(self.write(*c))).last();
+        let _ = s.as_bytes().iter().map(|c| block!(self.write(*c))).next_back();
         Ok(())
     }
 }
@@ -194,7 +194,7 @@ where
     Tx<USART>: hal::serial::Write<u8>,
 {
     fn write_str(&mut self, s: &str) -> fmt::Result {
-        let _ = s.as_bytes().iter().map(|c| block!(self.write(*c))).last();
+        let _ = s.as_bytes().iter().map(|c| block!(self.write(*c))).next_back();
         Ok(())
     }
 }