Treat MCC and MNC as strings, not integers

A MNC of 02 and 002 are *not* equal.  The former is a two-digit MNC
and the latter is a three-digit MNC.  Hence, we shouldn't treat
MNCs as integer values as we have no clue how many leading zeroes
(if any) the user entered.

Change-Id: I9d1d07a64888c76703c3e430bbdd822080c05819
Closes: OS#4523

pySim/utils.py

@@ -102,7 +102,9 @@ def enc_iccid(iccid):
 
 def enc_plmn(mcc, mnc):
 	"""Converts integer MCC/MNC into 3 bytes for EF"""
-	return swap_nibbles(lpad('%d' % int(mcc), 3) + lpad('%d' % int(mnc), 3))
+	if len(mnc) == 2:
+		mnc = "F%s" % mnc
+	return swap_nibbles("%s%s" % (mcc, mnc))
 
 def dec_spn(ef):
 	byte1 = int(ef[0:2])