bsc: Fix encoding of 2-digit MNC (paging tests)

The TTCN-3 data types are abstract data types,  Encoding artefacts
like 'F' for padding shouldn't be seen by the user.  Hence, let's
pass a 2-digit-long or 3-digit-long hexstring into the encoder
functions and let them determine if they should introduce any 'F'
for padding or not.

Change-Id: If4d3dfc16381493d7e710be746ed963975051fc1
diff --git a/library/BSSMAP_Templates.ttcn b/library/BSSMAP_Templates.ttcn
index 5d2bff2..5d13899 100644
--- a/library/BSSMAP_Templates.ttcn
+++ b/library/BSSMAP_Templates.ttcn
@@ -665,8 +665,8 @@
 }
 
 private function f_enc_mcc_mnc(GsmMcc mcc, GsmMnc mnc) return OCT3 {
-	if (mnc[0] == 'f'H) {
-		return hex2oct(mcc[1] & mcc[0] & mnc[0] & mcc[2] & mnc[2] & mnc[1]);
+	if (lengthof(mnc) == 2) {
+		return hex2oct(mcc[1] & mcc[0] & 'F'H & mcc[2] & mnc[1] & mnc[0]);
 	} else {
 		return hex2oct(mcc[1] & mcc[0] & mnc[2] & mcc[2] & mnc[1] & mnc[0]);
 	}