cosmetic: rename gsm_subscriber_connection->conn_fsm to ->fi

Match osmo-bsc's naming of the subscriber connection's FSM instance; 'conn->fi'
makes more sense anyway than 'conn->conn_fsm'.

BTW, an upcoming commit will do away with the legacy from libbsc/libmsc duality
and firmly glue the conn allocation to the fi.

Related: OS#3122
Change-Id: If442f2ba78d9722b1065ec30c9a13f372b6a8caa
diff --git a/include/osmocom/msc/gsm_data.h b/include/osmocom/msc/gsm_data.h
index 1e5d567..df15eea 100644
--- a/include/osmocom/msc/gsm_data.h
+++ b/include/osmocom/msc/gsm_data.h
@@ -75,6 +75,9 @@
 	/* global linked list of subscriber_connections */
 	struct llist_head entry;
 
+	/* FSM instance to control the subscriber connection's permissions and lifetime. */
+	struct osmo_fsm_inst *fi;
+
 	/* usage count. If this drops to zero, we start the release
 	 * towards A/Iu */
 	uint32_t use_count;
@@ -92,8 +95,6 @@
 	/* SMS helpers for libmsc */
 	uint8_t next_rp_ref;
 
-	struct osmo_fsm_inst *conn_fsm;
-
 	/* Are we part of a special "silent" call */
 	int silent_call;