Change serial_fd log from error to warn

This commit changes serial_FD log levels
from error to warn.

Signed-off-by: berciuliviu <lberciu@amazon.com>

src/devices/src/legacy/serial.rs

@@ -464,11 +464,11 @@ impl MutEventSubscriber for Serial {
                 let serial_fd = self.serial_input_fd();
                 if serial_fd != -1 {
                     if let Err(e) = ops.add(Events::new(&serial_fd, EventSet::IN)) {
-                        error!("Failed to register serial input fd: {}", e);
+                        warn!("Failed to register serial input fd: {}", e);
                     }
                 }
                 if let Err(e) = ops.add(Events::new(buf_ready_evt, EventSet::IN)) {
-                    error!("Failed to register serial buffer ready event: {}", e);
+                    warn!("Failed to register serial buffer ready event: {}", e);
                 }
             }
         }