bts: Add backpointers to the PDCH and TRX structures

This will allow to kill various parameters from all the functions
as we can walk back.
diff --git a/src/bts.cpp b/src/bts.cpp
index d6ed8f3..403b1e3 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -64,6 +64,19 @@
 	INIT_LLIST_HEAD(&m_bts.ul_tbfs);
 	INIT_LLIST_HEAD(&m_bts.dl_tbfs);
 	m_bts.bts = this;
+
+	/* initialize back pointers */
+	for (size_t trx_no = 0; trx_no < ARRAY_SIZE(m_bts.trx); ++trx_no) {
+		struct gprs_rlcmac_trx *trx = &m_bts.trx[trx_no];
+		trx->trx_no = trx_no;
+		trx->bts = this;
+
+		for (size_t ts_no = 0; ts_no < ARRAY_SIZE(trx->pdch); ++ts_no) {
+			struct gprs_rlcmac_pdch *pdch = &trx->pdch[ts_no];
+			pdch->ts_no = ts_no;
+			pdch->trx = trx;
+		}
+	}
 }
 
 void BTS::set_current_frame_number(int fn)