llc: Move the decision if a frame has expired into the LLC

This way the generation of the expiry information and the check
is at the same place. This should make reading the code more easy.
diff --git a/src/llc.cpp b/src/llc.cpp
index 2997ec1..c0c517f 100644
--- a/src/llc.cpp
+++ b/src/llc.cpp
@@ -102,3 +102,12 @@
 
 	timeradd(&now, &csec, tv);
 }
+
+bool gprs_llc::is_frame_expired(struct timeval *tv_now, struct timeval *tv)
+{
+	/* Timeout is infinite */
+	if (tv->tv_sec == 0 && tv->tv_usec == 0)
+		return false;
+
+	return timercmp(tv_now, tv, >);
+}