Use crc32_ccitt_tvb_offset_seed().

One fewer tvb_get_ptr() call.  Yay!

Change-Id: I08ac4888d696f0b6b6a17e9e22f3dfec5aedad59
Reviewed-on: https://code.wireshark.org/review/3496
Reviewed-by: Guy Harris <guy@alum.mit.edu>