Fix the polynomial for the CCITT CRC.

And note that it's the same polynomial for the MPEG-2 CRC.

Change-Id: Ie89e392156ae77a2adeec3eb8e704aa75c0cd0dc
Reviewed-on: https://code.wireshark.org/review/3512
Reviewed-by: Guy Harris <guy@alum.mit.edu>