Expect ms object to exist before calling tbf_alloc_dl_tbf()

Same as previous commit, this time for the DL counterpart.

Change-Id: I87f6cdf8288a688466020bda0874e68b57aa71c4
diff --git a/src/tbf.h b/src/tbf.h
index 33d996f..5ebd2e8 100644
--- a/src/tbf.h
+++ b/src/tbf.h
@@ -351,8 +351,7 @@
 
 struct gprs_rlcmac_ul_tbf *tbf_alloc_ul_tbf(struct gprs_rlcmac_bts *bts, GprsMs *ms, int8_t use_trx, bool single_slot);
 
-struct gprs_rlcmac_dl_tbf *tbf_alloc_dl_tbf(struct gprs_rlcmac_bts *bts, GprsMs *ms, int8_t use_trx, uint8_t ms_class,
-					    uint8_t egprs_ms_class, bool single_slot);
+struct gprs_rlcmac_dl_tbf *tbf_alloc_dl_tbf(struct gprs_rlcmac_bts *bts, GprsMs *ms, int8_t use_trx, bool single_slot);
 
 void tbf_free(struct gprs_rlcmac_tbf *tbf);