MT#61856 call.h: make `packet_stream_last_packet()` inliner consistent

With timestamps handling, which want to operate on signed integers.
Return the signed max value given from the calls of `atomic64_get_na()`.
Raw-cast the output of `atomic64_get_na()` to the signed integer as well.

Change-Id: Icc6878c9b08545f456ad5eb267ed6a3333c11ab7

include/call.h

@@ -464,8 +464,8 @@ struct packet_stream {
 };
 
 INLINE int64_t packet_stream_last_packet(const struct packet_stream *ps) {
-	uint64_t lp1 = atomic64_get_na(&ps->last_packet_us);
-	uint64_t lp2 = atomic64_get_na(&ps->stats_in->last_packet_us);
+	int64_t lp1 = (int64_t)atomic64_get_na(&ps->last_packet_us);
+	int64_t lp2 = (int64_t)atomic64_get_na(&ps->stats_in->last_packet_us);
 	return MAX(lp1, lp2);
 }