tbf_dl_fsm_ctx: Properly define tbf backptr as dl_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: Id2571e55e1fea2918207175f2030ec026e880bc1
diff --git a/src/tbf_dl.cpp b/src/tbf_dl.cpp
index ae90041..f5637f9 100644
--- a/src/tbf_dl.cpp
+++ b/src/tbf_dl.cpp
@@ -181,7 +181,7 @@
 	m_dl_egprs_ctrs(NULL)
 {
 	memset(&state_fsm, 0, sizeof(state_fsm));
-	state_fsm.tbf = (struct gprs_rlcmac_tbf *)this;
+	state_fsm.dl_tbf = this;
 	state_fi = osmo_fsm_inst_alloc(&tbf_dl_fsm, this, &state_fsm, LOGL_INFO, NULL);
 	OSMO_ASSERT(state_fi);
 }