sysmocom_sja2: simplify and fix op/opc decoder/encoder

The decoder/encoder of that decodes the EF.xSIM_AUTH_KEY files has an
overcomplicated handling for op/and opc. There is a condition that
checks if milenage is configured and another one that checks if the
string is recognized as OP or OPc. Both is not correct and seems not to
work (op and opc is always displayed as "null")

The encoder/decoder should focus on the physical file layout and
regardless of any other conriguration the OP/OPc field is physically
present and should be displayd and presented for editing.

Change-Id: I6fa3a07e5e473273498d3f13d4cfa33743b787e1
diff --git a/pySim/sysmocom_sja2.py b/pySim/sysmocom_sja2.py
index 53e6585..a78318e 100644
--- a/pySim/sysmocom_sja2.py
+++ b/pySim/sysmocom_sja2.py
@@ -143,12 +143,7 @@
                             'algorithm'/Enum(Nibble, milenage=4, comp128v1=1, comp128v2=2, comp128v3=3))
         self._construct = Struct('cfg'/CfgByte,
                                  'key'/HexAdapter(Bytes(16)),
-                                 'op'/ If(this.cfg.algorithm == 'milenage' and not this.cfg.use_opc_instead_of_op,
-                                     HexAdapter(Bytes(16))),
-                                 'opc' /
-                                 If(this.cfg.algorithm == 'milenage' and this.cfg.use_opc_instead_of_op,
-                                     HexAdapter(Bytes(16)))
-                                 )
+                                 'op_opc' /HexAdapter(Bytes(16)))
 
 
 class DF_SYSTEM(CardDF):
@@ -198,13 +193,7 @@
                             'algorithm'/Enum(Nibble, milenage=4, sha1_aka=5, xor=15))
         self._construct = Struct('cfg'/CfgByte,
                                  'key'/HexAdapter(Bytes(16)),
-                                 'op' /
-                                 If(this.cfg.algorithm == 'milenage' and not this.cfg.use_opc_instead_of_op,
-                                     HexAdapter(Bytes(16))),
-                                 'opc' /
-                                 If(this.cfg.algorithm == 'milenage' and this.cfg.use_opc_instead_of_op,
-                                     HexAdapter(Bytes(16)))
-                                 )
+                                 'op_opc' /HexAdapter(Bytes(16)))
 
 
 class EF_USIM_AUTH_KEY_2G(TransparentEF):
@@ -216,13 +205,7 @@
                             'algorithm'/Enum(Nibble, milenage=4, comp128v1=1, comp128v2=2, comp128v3=3))
         self._construct = Struct('cfg'/CfgByte,
                                  'key'/HexAdapter(Bytes(16)),
-                                 'op' /
-                                 If(this.cfg.algorithm == 'milenage' and not this.cfg.use_opc_instead_of_op,
-                                     HexAdapter(Bytes(16))),
-                                 'opc' /
-                                 If(this.cfg.algorithm == 'milenage' and this.cfg.use_opc_instead_of_op,
-                                     HexAdapter(Bytes(16)))
-                                 )
+                                 'op_opc' /HexAdapter(Bytes(16)))
 
 
 class EF_GBA_SK(TransparentEF):