bts: Fix typo in field name

Change-Id: I5426ff4ccbc45464888e2246cceb8e861d1e477e
diff --git a/src/bts.cpp b/src/bts.cpp
index dc550ef..95dde04 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -1332,7 +1332,7 @@
 	if (bts->pcu->vty.force_alpha != (uint8_t)-1)
 		return bts->pcu->vty.force_alpha;
 	if (bts->si13_is_set)
-		return bts->si31_ro_decoded.pwr_ctrl_pars.alpha;
+		return bts->si13_ro_decoded.pwr_ctrl_pars.alpha;
 	/* default if no SI13 is received yet: closed loop control, TS 44.060
 	 * B.2 Closed loop control */
 	return 0;