tbf_ul_fsm_ctx: Properly define tbf backptr as ul_tbf

Since the tbf_fsm was split recently into tbf_dl_fsm and tbf_ul_fsm,
each has now its own ctx strucvture, which can hold the proper tbf
subclass.

Change-Id: I7741d524a14437caf4c92b9c09e19762eb272e30
diff --git a/src/tbf_ul.cpp b/src/tbf_ul.cpp
index 246c0e1..43cd4f6 100644
--- a/src/tbf_ul.cpp
+++ b/src/tbf_ul.cpp
@@ -202,7 +202,7 @@
 	memset(&m_usf, USF_INVALID, sizeof(m_usf));
 
 	memset(&state_fsm, 0, sizeof(state_fsm));
-	state_fsm.tbf = (struct gprs_rlcmac_tbf *)this;
+	state_fsm.ul_tbf = this;
 	state_fi = osmo_fsm_inst_alloc(&tbf_ul_fsm, this, &state_fsm, LOGL_INFO, NULL);
 	OSMO_ASSERT(state_fi);