tbf: Have one imsi field and assign it through a function

Have one IMSI field per TBF and assign through a function call.
The IMSI should be used to look-up the TBF on the SGSN->PCU
direction.
diff --git a/src/gprs_rlcmac_meas.cpp b/src/gprs_rlcmac_meas.cpp
index 5bb74cd..dd9de5a 100644
--- a/src/gprs_rlcmac_meas.cpp
+++ b/src/gprs_rlcmac_meas.cpp
@@ -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->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->imsi(), tbf->tlli(),
 		tbf->meas.dl_bw_octets / elapsed);
 
 	/* reset bandwidth values timestamp */