mgcp_common: rename to MGCP_CONN_ID_MAXLEN

So far, MGCP_CONN_ID_LENGTH was often used as exactly the length of the
Connection Identifier. To indicate this length as a maximum, introduce the
MGCP_CONN_ID_MAXLEN and use it everywhere. Keep the old name as an alias.

Change-Id: I1117003c7614e98535d5c201d002e459c01bdc3f
diff --git a/src/libosmo-mgcp-client/mgcp_client_fsm.c b/src/libosmo-mgcp-client/mgcp_client_fsm.c
index 5368ec3..da900c5 100644
--- a/src/libosmo-mgcp-client/mgcp_client_fsm.c
+++ b/src/libosmo-mgcp-client/mgcp_client_fsm.c
@@ -41,7 +41,7 @@
 	bool mgw_trans_pending;
 
 	/* Connection ID which has been assigned by he MGW */
-	char conn_id[MGCP_CONN_ID_LENGTH];
+	char conn_id[MGCP_CONN_ID_MAXLEN];
 
 	/* Local RTP connection info, the MGW will send outgoing traffic to the
 	 * ip/port specified here. The Address does not have to be choosen right