nat: Store the msc_con in the data of the bsc_fd

* Stop using nat->msc_con in the read_cb but use the data.
diff --git a/openbsc/include/openbsc/bsc_msc.h b/openbsc/include/openbsc/bsc_msc.h
index faf6faa..72e376c 100644
--- a/openbsc/include/openbsc/bsc_msc.h
+++ b/openbsc/include/openbsc/bsc_msc.h
@@ -30,6 +30,7 @@
 	struct write_queue write_queue;
 	int is_connected;
 	int is_authenticated;
+	int first_contact;
 	const char *ip;
 	int port;
 	int prio;