tbf/test: Fix old_tbf argument to tbf_alloc_ul_tbf

Currently the ul_tbf is used in its own declaration as an argument to
tbf_alloc_ul_tbf, where dl_tbf can be used instead and makes more
sense.

This commit uses dl_tbf instead of ul_tbf as old_tbf.

Addresses:
  tbf/TbfTest.cpp:71:14: warning: ‘ul_tbf’ may be used uninitialized in
      this function

Sponsored-by: On-Waves ehf
diff --git a/tests/tbf/TbfTest.cpp b/tests/tbf/TbfTest.cpp
index 51c4c88..9d01636 100644
--- a/tests/tbf/TbfTest.cpp
+++ b/tests/tbf/TbfTest.cpp
@@ -67,7 +67,7 @@
 	the_bts.timing_advance()->remember(0x2342, dl_tbf->ta);
 
 	gprs_rlcmac_tbf *ul_tbf = tbf_alloc_ul_tbf(the_bts.bts_data(),
-						ul_tbf, 0,
+						dl_tbf, 0,
 						0, 0, 0);
 	ul_tbf->update_tlli(0x2342);
 	ul_tbf->tlli_mark_valid();