How about

def format_hex_bits(bits):
  pass # implement this and others

type_to_wireshark_type = {
  Types.hex: format_hex_bits,
  # other number types
  Types.string: 'FT_STRING',
...
}

Reasoning: Pulling conversions logic to their own functions (or one function) will make code easier to read. Also, this function is a dict with just a custom exception instead of KeyError. Using plain dict is imo clearer, also bit more succint.