bts, tbf: Split alloc_tbf function into separate UL and DL versions

UL and DL tbfs are used in very separate parts and are not the same
thing so split the alloc function and use the UL/DL version throughout
the code.

Ticket: SYS#389
Sponsored-by: On-Waves ehf
diff --git a/tests/tbf/TbfTest.cpp b/tests/tbf/TbfTest.cpp
index 38975f9..decf4d9 100644
--- a/tests/tbf/TbfTest.cpp
+++ b/tests/tbf/TbfTest.cpp
@@ -44,16 +44,16 @@
 	/*
 	 * Make a uplink and downlink allocation
 	 */
-	gprs_rlcmac_tbf *dl_tbf = tbf_alloc(the_bts.bts_data(),
-						NULL, GPRS_RLCMAC_DL_TBF, 0,
+	gprs_rlcmac_tbf *dl_tbf = tbf_alloc_dl_tbf(the_bts.bts_data(),
+						NULL, 0,
 						0, 0, 0);
 	dl_tbf->update_tlli(0x2342);
 	dl_tbf->tlli_mark_valid();
 	dl_tbf->ta = 4;
 	the_bts.timing_advance()->remember(0x2342, dl_tbf->ta);
 
-	gprs_rlcmac_tbf *ul_tbf = tbf_alloc(the_bts.bts_data(),
-						ul_tbf, GPRS_RLCMAC_UL_TBF, 0,
+	gprs_rlcmac_tbf *ul_tbf = tbf_alloc_ul_tbf(the_bts.bts_data(),
+						ul_tbf, 0,
 						0, 0, 0);
 	ul_tbf->update_tlli(0x2342);
 	ul_tbf->tlli_mark_valid();