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/bts.h b/src/bts.h
index d2b1f9f..085b448 100644
--- a/src/bts.h
+++ b/src/bts.h
@@ -301,7 +301,7 @@
 struct gprs_rlcmac_dl_tbf *bts_dl_tbf_by_tfi(struct gprs_rlcmac_bts *bts, uint8_t tfi, uint8_t trx, uint8_t ts);
 struct gprs_rlcmac_ul_tbf *bts_ul_tbf_by_tfi(struct gprs_rlcmac_bts *bts, uint8_t tfi, uint8_t trx, uint8_t ts);
 
-void bts_snd_dl_ass(struct gprs_rlcmac_bts *bts, struct gprs_rlcmac_tbf *tbf);
+void bts_snd_dl_ass(struct gprs_rlcmac_bts *bts, const struct gprs_rlcmac_dl_tbf *tbf);
 
 void bts_set_current_frame_number(struct gprs_rlcmac_bts *bts, uint32_t frame_number);
 void bts_set_current_block_frame_number(struct gprs_rlcmac_bts *bts, int frame_number);