gsm_sms: Use ENC_BIG_ENDIAN when fetching FT_U?INT8 fields

Change-Id: If41cf6c10e4af8554cd8f6a69bd8654ad7262c85
Reviewed-on: https://code.wireshark.org/review/5749
Reviewed-by: Bill Meier <wmeier@newsguy.com>