Silence coverity warning about dead code
    
    For some reason, coverity was complaining that the definition of
    cred_type_to_str was dead code, even though it wasn't. Changing the
    storage to static silences the warning. Since that's a benficial change
    anyway, let's change it. At the same time, make the pointer constant as
    well and move it outside of the loop since it doesn't need to be inside
    the loop.
    
    Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
    Acked-by: Eduardo Lima (Etrunko) <etrunko@redhat.com>