cosmetic: use const pointer for bts_data

It's used several time for logging so let's call it once to make code
easier to follow.

Change-Id: Icfd9e5603a5d8701f487f17e9c0335d458e9e80b
diff --git a/src/tbf_dl.cpp b/src/tbf_dl.cpp
index c97436a..d5e4a45 100644
--- a/src/tbf_dl.cpp
+++ b/src/tbf_dl.cpp
@@ -1350,9 +1350,11 @@
 
 void gprs_rlcmac_dl_tbf::set_window_size()
 {
-	uint16_t ws = egprs_window_size(bts->bts_data(), dl_slots());
+	const struct gprs_rlcmac_bts *b = bts->bts_data();
+	uint16_t ws = egprs_window_size(b, dl_slots());
+
 	LOGPTBFDL(this, LOGL_INFO, "setting EGPRS DL window size to %u, base(%u) slots(%u) ws_pdch(%u)\n",
-		  ws, bts->bts_data()->ws_base, pcu_bitcount(dl_slots()), bts->bts_data()->ws_pdch);
+		  ws, b->ws_base, pcu_bitcount(dl_slots()), b->ws_pdch);
 	m_window.set_ws(ws);
 }
 
diff --git a/src/tbf_ul.cpp b/src/tbf_ul.cpp
index fd7d73b..e1a0fbd 100644
--- a/src/tbf_ul.cpp
+++ b/src/tbf_ul.cpp
@@ -588,8 +588,9 @@
 
 void gprs_rlcmac_ul_tbf::set_window_size()
 {
-	uint16_t ws = egprs_window_size(bts->bts_data(), ul_slots());
+	const struct gprs_rlcmac_bts *b = bts->bts_data();
+	uint16_t ws = egprs_window_size(b, ul_slots());
 	LOGPTBFUL(this, LOGL_INFO, "setting EGPRS UL window size to %u, base(%u) slots(%u) ws_pdch(%u)\n",
-		  ws, bts->bts_data()->ws_base, pcu_bitcount(ul_slots()), bts->bts_data()->ws_pdch);
+		  ws, b->ws_base, pcu_bitcount(ul_slots()), b->ws_pdch);
 	m_window.set_ws(ws);
 }