filesystem: Use Tuple for record length

The size should be a *tuple*.  In reality we so far passed a set.  The
problem with the set is that ordering is not guaranteed, and hence we
cannot assume the first and second item have meaning (minimum vs.
default record length).

Change-Id: I470f4e69c83cb2761861b3350bf8d49e31f4d957
diff --git a/pySim/ts_102_221.py b/pySim/ts_102_221.py
index 6e04e96..83ddb85 100644
--- a/pySim/ts_102_221.py
+++ b/pySim/ts_102_221.py
@@ -588,7 +588,7 @@
         pass
 
     def __init__(self, fid='2f00', sfid=0x1e, name='EF.DIR', desc='Application Directory'):
-        super().__init__(fid, sfid=sfid, name=name, desc=desc, rec_len={5, 54})
+        super().__init__(fid, sfid=sfid, name=name, desc=desc, rec_len=(5, 54))
         self._tlv = EF_DIR.ApplicationTemplate
 
 # TS 102 221 Section 13.2