tbf.h: Fix wrong type declaration of field dl_ass_fsm

The typo was harmless since as of now the two structs where identical.

Change-Id: Ibbc59a0584ee0cd83de0e04eebdcca2aebe3e970
diff --git a/src/tbf.h b/src/tbf.h
index 97f5562..4170ca1 100644
--- a/src/tbf.h
+++ b/src/tbf.h
@@ -245,7 +245,7 @@
 	struct rate_ctr_group *m_ctrs;
 	struct osmo_fsm_inst *state_fi;
 	struct tbf_ul_ass_fsm_ctx ul_ass_fsm;
-	struct tbf_ul_ass_fsm_ctx dl_ass_fsm;
+	struct tbf_dl_ass_fsm_ctx dl_ass_fsm;
 
 	struct llist_item m_ms_list;
 	struct llist_item m_trx_list;