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/src/bts.cpp b/src/bts.cpp
index 0986ed8..ff1a065 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -461,7 +461,7 @@
 			return -EBUSY;
 		}
 		/* set class to 0, since we don't know the multislot class yet */
-		tbf = (gprs_rlcmac_ul_tbf *)tbf_alloc(&m_bts, NULL, GPRS_RLCMAC_UL_TBF, tfi, trx_no, 0, 1);
+		tbf = tbf_alloc_ul_tbf(&m_bts, NULL, tfi, trx_no, 0, 1);
 		if (!tbf) {
 			LOGP(DRLCMAC, LOGL_NOTICE, "No PDCH resource\n");
 			/* FIXME: send reject */