cards: use string representation for MNC/MCC

At the moment MNC and MCC are represented as integer numbers inside the
parameter array while all other parameters are represented as strings.
Lets use strings for MNC/MCC as well to simplify the parameter handling.
We will also not loose the length information in case of leading zeros.

Change-Id: Ia2333921a4863f0f26ee923ca796e62ec5e2d59a
diff --git a/pySim/utils.py b/pySim/utils.py
index 65f10c5..a68af0a 100644
--- a/pySim/utils.py
+++ b/pySim/utils.py
@@ -98,7 +98,7 @@
 
 def enc_plmn(mcc, mnc):
 	"""Converts integer MCC/MNC into 3 bytes for EF"""
-	return swap_nibbles(lpad('%d' % mcc, 3) + lpad('%d' % mnc, 3))
+	return swap_nibbles(lpad('%d' % int(mcc), 3) + lpad('%d' % int(mnc), 3))
 
 def dec_spn(ef):
 	byte1 = int(ef[0:2])