Use ENC_LITTLE_ENDIAN when fetching FT_U?INT8 fields ...

  (for some dissectors which fetch all other integral fields using
   ENC_LITTLE_ENDIAN).

Change-Id: Ica72a68ac560f2920d61e0769de83130557c46fd
Reviewed-on: https://code.wireshark.org/review/5752
Reviewed-by: Bill Meier <wmeier@newsguy.com>