tbf: Make the tlli "private" and update the updating code

Now all updates to the tlli/tlli_valid are in one place. If we
implement the policy to update the matching/linked TBF we can
now to do it in a single place. Add a todo item for that as I
am waiting for feedback from the mailinglist.
diff --git a/src/gprs_rlcmac_meas.cpp b/src/gprs_rlcmac_meas.cpp
index 3229795..5bb74cd 100644
--- a/src/gprs_rlcmac_meas.cpp
+++ b/src/gprs_rlcmac_meas.cpp
@@ -100,7 +100,7 @@
 		return -EINVAL;
 
 	LOGP(DRLCMACMEAS, LOGL_INFO, "UL RSSI of TLLI=0x%08x: %d dBm\n",
-		tbf->tlli, tbf->meas.rssi_sum / tbf->meas.rssi_num);
+		tbf->tlli(), tbf->meas.rssi_sum / tbf->meas.rssi_num);
 
 	return 0;
 }
@@ -123,7 +123,7 @@
 		return -EINVAL;
 
 	LOGP(DRLCMACMEAS, LOGL_DEBUG, "DL Loss of TLLI 0x%08x: Received: %4d  "
-		"Lost: %4d  Sum: %4d\n", tbf->tlli, received, lost, sum);
+		"Lost: %4d  Sum: %4d\n", tbf->tlli(), received, lost, sum);
 
 	tbf->meas.dl_loss_received += received;
 	tbf->meas.dl_loss_lost += lost;
@@ -154,7 +154,7 @@
 		return -EINVAL;
 
 	LOGP(DRLCMACMEAS, LOGL_INFO, "DL packet loss of IMSI=%s / TLLI=0x%08x: "
-		"%d%%\n", tbf->meas.imsi, tbf->tlli,
+		"%d%%\n", tbf->meas.imsi, tbf->tlli(),
 		tbf->meas.dl_loss_lost * 100 / sum);
 
 	return 0;
@@ -179,7 +179,7 @@
 		return 0;
 
 	LOGP(DRLCMACMEAS, LOGL_INFO, "DL Bandwitdh of IMSI=%s / TLLI=0x%08x: "
-		"%d KBits/s\n", tbf->meas.imsi, tbf->tlli,
+		"%d KBits/s\n", tbf->meas.imsi, tbf->tlli(),
 		tbf->meas.dl_bw_octets / elapsed);
 
 	/* reset bandwidth values timestamp */