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_api.c b/openbsc/src/osmo-bsc/osmo_bsc_api.c
index 5a01d6b..00a10b3 100644
--- a/openbsc/src/osmo-bsc/osmo_bsc_api.c
+++ b/openbsc/src/osmo-bsc/osmo_bsc_api.c
@@ -53,8 +53,8 @@
 
 static uint16_t get_network_code_for_msc(struct osmo_msc_data *msc)
 {
-	if (msc->core_ncc != -1)
-		return msc->core_ncc;
+	if (msc->core_mnc != -1)
+		return msc->core_mnc;
 	return msc->network->network_code;
 }