The "MPEG-2" CRC-32 is also the ATM CRC-32.

Use it in the ATM dissector, and use a tvbuff version, so that we don't
do tvb_get_ptr() ourselves.

Change-Id: I0bd3594bc739e0cca447ac06f34a471441cf2e70
Reviewed-on: https://code.wireshark.org/review/3513
Reviewed-by: Guy Harris <guy@alum.mit.edu>