Use ENC_BIG_ENDIAN when fetching FT_U?INT8 fields ...

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

Change-Id: Ic18e3172aad76af12b12d6732c88497be22aed56
Reviewed-on: https://code.wireshark.org/review/5748
Reviewed-by: Bill Meier <wmeier@newsguy.com>