ms: Fix and use ms msisdn() getter

The getter method was named the same as the itnernal field, and hence
when used it would fail since the intenral field would be sleect and
fail to be called.

Change-Id: I2f631eb6256eb0e065f41d5b7531395c4a054cd8
diff --git a/sysmocom/suites/encryption/lib/testlib.py b/sysmocom/suites/encryption/lib/testlib.py
index 87581c6..e85e1c9 100644
--- a/sysmocom/suites/encryption/lib/testlib.py
+++ b/sysmocom/suites/encryption/lib/testlib.py
@@ -45,7 +45,7 @@
         hlr.subscriber_delete(ms)
         print('KI changed: ' + bad_ki + " => " + good_ki)
         ms.set_ki(good_ki)
-        hlr.subscriber_add(ms, ms.msisdn)
+        hlr.subscriber_add(ms, ms.msisdn())
         print('Attempt connection with correct KI...')
     else:
         print('Attempt connection with wrong KI, should work as it is not used...')