Use expert info to report unknown TLV types.

Unless we plan to fill in all 256 entries in the TLV type table, and
unless that's because all 256 types are valid, we should report unknown
types with an expert info, not a dissector assertion.

Change-Id: Ia6fca37ad6a5969caad3229abaf08a25e20ba992
Reviewed-on: https://code.wireshark.org/review/3707
Reviewed-by: Guy Harris <guy@alum.mit.edu>