Add a tvbuff version of crc6_compute().

Use it in the MBMS synchronisation protocol dissector, rather than
calling tvb_get_ptr() there.

Change-Id: I7ddb3c6b30547826cb5372352c7c483d8a24dc8e
Reviewed-on: https://code.wireshark.org/review/3514
Reviewed-by: Guy Harris <guy@alum.mit.edu>