tbf_ul: Fix wrong struct type in field

This typo was harmless so far because both old and new structures have
the exact same content.

Change-Id: If2e20128a589a25b9dc3fd08340cdb92aad158c3
diff --git a/src/tbf_ul.h b/src/tbf_ul.h
index 38784f8..c5c55f7 100644
--- a/src/tbf_ul.h
+++ b/src/tbf_ul.h
@@ -105,7 +105,7 @@
 	struct rate_ctr_group *m_ul_egprs_ctrs;
 
 	struct tbf_ul_fsm_ctx state_fsm;
-	struct tbf_ul_ass_fsm_ctx ul_ack_fsm;
+	struct tbf_ul_ack_fsm_ctx ul_ack_fsm;
 
 protected:
 	void maybe_schedule_uplink_acknack(const gprs_rlc_data_info *rlc, bool countdown_finished);