Rename as_{dl,ul}_tbf -> tbf_as_{dl,ul}_tbf

Prefix the function accordingly to specify the input object.

Change-Id: I225568fe2cb5236817cdd9609354c24c66723606
diff --git a/src/tbf_fsm.c b/src/tbf_fsm.c
index 2189747..2b712e7 100644
--- a/src/tbf_fsm.c
+++ b/src/tbf_fsm.c
@@ -342,7 +342,7 @@
 
 static void handle_timeout_X2002(struct tbf_fsm_ctx *ctx)
 {
-	struct gprs_rlcmac_dl_tbf *dl_tbf = as_dl_tbf(ctx->tbf);
+	struct gprs_rlcmac_dl_tbf *dl_tbf = tbf_as_dl_tbf(ctx->tbf);
 
 	/* X2002 is used only for DL TBF */
 	OSMO_ASSERT(dl_tbf);