packet-gsm_cbch.c: various minor changes

- tvb_length() --> tvb_reported_length();
- Use -1 for length arg in proto_tree_add_protocol_format();
- Remove unused 'for' loop increment which caused Windows
  warning: "C4701: potentially uninitialized local variable";
- Add editor modelines.

Change-Id: I37833b750c100e333f4fa76d9465169be456d7c1
Reviewed-on: https://code.wireshark.org/review/3788
Reviewed-by: Bill Meier <wmeier@newsguy.com>