fix measurement report parsing of MEAS VALID logic

As Dieter points out, the logic level is inverted. 0 means valid.
diff --git a/openbsc/src/gsm_04_08.c b/openbsc/src/gsm_04_08.c
index 2312e8a..4ec713d 100644
--- a/openbsc/src/gsm_04_08.c
+++ b/openbsc/src/gsm_04_08.c
@@ -240,7 +240,7 @@
 		rep->flags |= MEAS_REP_F_BA1;
 	if (data[0] & 0x40)
 		rep->flags |= MEAS_REP_F_DTX;
-	if (data[1] & 0x40)
+	if ((data[1] & 0x40) == 0x00)
 		rep->flags |= MEAS_REP_F_VALID;
 
 	rep->rxlev_full = data[0] & 0x3f;