tbf/bts: Rename tbf->snd_dl_ack to tbf->rcvd_dl_ack

This function is called to act upon a received DL ACK packet so this
name makes more sense.
diff --git a/src/tbf.cpp b/src/tbf.cpp
index d6b3802..b20d0fc 100644
--- a/src/tbf.cpp
+++ b/src/tbf.cpp
@@ -1425,7 +1425,7 @@
 	return 0;
 }
 
-int gprs_rlcmac_tbf::snd_dl_ack(uint8_t final_ack, uint8_t ssn, uint8_t *rbb)
+int gprs_rlcmac_tbf::rcvd_dl_ack(uint8_t final_ack, uint8_t ssn, uint8_t *rbb)
 {
 	LOGP(DRLCMACDL, LOGL_DEBUG, "%s downlink acknowledge\n", tbf_name(this));