packet-infiniband: added support for Mellanox DC transport type

*marked parse_DCCETH() function's unesed parameters with _U_ macro instead of (void) */
*Added few whitespaces (new lines), were mistakenly removed"*

Adding minimal support for Mellanox DC transport type which
replaces RD (RD isn't implemented in any HW yet).
It's minimal in the sense of skipping new headers without
parsing/showing them, this allows at least to keep correct IB packet parsing
due to the new added header for this TS. (in this stage we can't expose
the protocol details including it's header layout)

Change-Id: I6cffa6497632ba19a6f9ec60a4f68da60b28e821
Reviewed-on: https://code.wireshark.org/review/3719
Reviewed-by: Evan Huus <eapache@gmail.com>
Petri-Dish: Evan Huus <eapache@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>