bsc: Rename core_ncc to core_mnc

Struct osmo_msc_data contains int core_ncc, which is actually the
MNC part of the PLMN, not to be confused with the Network Colour
Code.

The following patch renames this field for clarity and consistency
with the standards.
diff --git a/openbsc/src/osmo-bsc/osmo_bsc_filter.c b/openbsc/src/osmo-bsc/osmo_bsc_filter.c
index 58a57cd..6058d7e 100644
--- a/openbsc/src/osmo-bsc/osmo_bsc_filter.c
+++ b/openbsc/src/osmo-bsc/osmo_bsc_filter.c
@@ -313,7 +313,7 @@
 
 static int has_core_identity(struct osmo_msc_data *msc)
 {
-	if (msc->core_ncc != -1)
+	if (msc->core_mnc != -1)
 		return 1;
 	if (msc->core_mcc != -1)
 		return 1;