tbf: Add tbf_as_{ul,dl}_tbf_const func helpers
This way we can easily get the subclass from the parent if the pointer
is const. Similar to what we already have for the opposite direction
{ul,dl}_tbf_as_tbf_const().
Change-Id: I35e650d13ecf3a5020d136e7d8d99837786503e2
diff --git a/src/tbf_dl.h b/src/tbf_dl.h
index bfe9068..3a7c41d 100644
--- a/src/tbf_dl.h
+++ b/src/tbf_dl.h
@@ -130,6 +130,7 @@
int8_t use_trx, bool single_slot);
struct gprs_rlcmac_dl_tbf *tbf_as_dl_tbf(struct gprs_rlcmac_tbf *tbf);
+const struct gprs_rlcmac_dl_tbf *tbf_as_dl_tbf_const(const struct gprs_rlcmac_tbf *tbf);
/* dispatch Unitdata.DL messages */
int dl_tbf_handle(struct gprs_rlcmac_bts *bts,
const uint32_t tlli, const uint32_t old_tlli,