make sctp::poll_timeout as &self

sctp/src/association/mod.rs

@@ -404,7 +404,7 @@ impl Association {
     /// - a call to `poll_transmit` returned `Some`
     /// - a call was made to `handle_timeout`
     #[must_use]
-    pub fn poll_timeout(&mut self) -> Option<Instant> {
+    pub fn poll_timeout(&self) -> Option<Instant> {
         self.timers.next_timeout()
     }