packet-gsm_a_dtap.c: various changes (mostly indentation/whitespace/formatting);

- Remove #include <epan/prefs.h> (no prefs);
- tvb_length() --> tvb_reported_length()
- Use '4 space' indentation;
- Use consistent whitespace formatting;
- Reformat hf[] entries for readability;

Change-Id: I4fd7e860efa795256972d6a31155f0502fb7dcfd
Reviewed-on: https://code.wireshark.org/review/3612
Reviewed-by: Bill Meier <wmeier@newsguy.com>