lchan: Remember why a channel is broken using static strings

Remember why a channel is being marked as broken. So we can
maybe understand what happend.
diff --git a/openbsc/src/libbsc/chan_alloc.c b/openbsc/src/libbsc/chan_alloc.c
index c86556b..d8f9087 100644
--- a/openbsc/src/libbsc/chan_alloc.c
+++ b/openbsc/src/libbsc/chan_alloc.c
@@ -198,6 +198,8 @@
 
 		/* clear multi rate config */
 		memset(&lchan->mr_conf, 0, sizeof(lchan->mr_conf));
+
+		lchan->broken_reason = "";
 	} else {
 		struct challoc_signal_data sig;
 		sig.bts = bts;