fix bug in timeout handling

use timeout.saturating_sub instead of plain '-'

src/modem.rs

@@ -150,7 +150,7 @@ impl<UART: serial::Uart> Modem<UART> {
         let match_text: String = contains.unwrap_or("\n".to_string());
 
         loop {
-            let timeout = timeout - start.elapsed();
+            let timeout = timeout.saturating_sub(start.elapsed());
             let line = self.rx.read_line(timeout)?;
             print!("Read {} bytes from serial: {}", line.len(), line);
             response.push_str(&line);