tbf: Make tfi private and update the code

All logging code that used tbf->tfi is now using tbf_name to
print the the TBF. External code is now using tfi() which is
inlined and should result in the same code being generated as
before (+debug code that can be stripped).
diff --git a/src/gprs_rlcmac_ts_alloc.cpp b/src/gprs_rlcmac_ts_alloc.cpp
index c92192c..8d8e7f0 100644
--- a/src/gprs_rlcmac_ts_alloc.cpp
+++ b/src/gprs_rlcmac_ts_alloc.cpp
@@ -120,8 +120,8 @@
 				struct gprs_rlcmac_pdch *pdch,
 				struct gprs_rlcmac_tbf *tbf, int8_t usf)
 {
-	tbf->trx->ul_tbf[tbf->tfi] = tbf;
-	pdch->ul_tbf[tbf->tfi] = tbf;
+	tbf->trx->ul_tbf[tbf->tfi()] = tbf;
+	pdch->ul_tbf[tbf->tfi()] = tbf;
 	tbf->pdch[pdch->ts_no] = pdch;
 	tbf->dir.ul.usf[pdch->ts_no] = usf;
 }
@@ -130,8 +130,8 @@
 				struct gprs_rlcmac_pdch *pdch,
 				struct gprs_rlcmac_tbf *tbf)
 {
-	tbf->trx->dl_tbf[tbf->tfi] = tbf;
-	pdch->dl_tbf[tbf->tfi] = tbf;
+	tbf->trx->dl_tbf[tbf->tfi()] = tbf;
+	pdch->dl_tbf[tbf->tfi()] = tbf;
 	tbf->pdch[pdch->ts_no] = pdch;
 }