cosmetic: Hide all accesses to conn->bts behind conn_get_bts()

This is a new inline function that hides all accesses to conn->bts.

A follow-up patch will then point this to conn->lchan->ts->trx->bts
to get rid of the bts field.

Change-Id: Ib6cf7097ced34eebe80441c29ab1534f21956a33
diff --git a/include/osmocom/bsc/gsm_data.h b/include/osmocom/bsc/gsm_data.h
index 3cc3794..45fbcd2 100644
--- a/include/osmocom/bsc/gsm_data.h
+++ b/include/osmocom/bsc/gsm_data.h
@@ -98,6 +98,10 @@
 	struct gsm_classmark classmark;
 };
 
+static inline struct gsm_bts *conn_get_bts(struct gsm_subscriber_connection *conn) {
+	return conn->bts;
+}
+
 
 #include "gsm_data_shared.h"