Have media dissector take its "media type parameter" from dissector data parameter instead of pinfo->private_data.

Change-Id: I0a0cb0b4838bc4e55a759fb6031355892c220c8e
Reviewed-on: https://code.wireshark.org/review/5461
Reviewed-by: Michael Mann <mmann78@netscape.net>
Petri-Dish: Michael Mann <mmann78@netscape.net>
Reviewed-by: Anders Broman <a.broman58@gmail.com>