mgcp: Remember if the endpoint was allocated...

Do not use the CI_UNUSED to decide if an endpoint is allocated
but introduce a new flag. This way only the CRCX and free_endp
play with the allocated field.
diff --git a/openbsc/src/mgcp/mgcp_network.c b/openbsc/src/mgcp/mgcp_network.c
index ce81f3c..96a1865 100644
--- a/openbsc/src/mgcp/mgcp_network.c
+++ b/openbsc/src/mgcp/mgcp_network.c
@@ -215,7 +215,7 @@
 	}
 
 	/* do not forward aynthing... maybe there is a packet from the bts */
-	if (endp->ci == CI_UNUSED)
+	if (!endp->allocated)
 		return -1;
 
 	#warning "Slight spec violation. With connection mode recvonly we should attempt to forward."