IANA assigned port number 6653 to OpenFlow. However, applications

IANA assigned port number 6653 to OpenFlow.  However, applications
(e.g. opendaylight, nox, openvswitch, etc) still use legacy port
numbers.  The most common are 6633 and 6634.  This patch adds a
simple heuristic logic and uses the current uint preference as
additional input.  In most cases no user intervention is needed and
OpenFlow is automatically detected/dissected.

Change-Id: Iebf09b7b870efe9d52421b9acc238208d25d4565
Signed-off-by: Francesco Fondelli <francesco.fondelli@gmail.com>
Reviewed-on: https://code.wireshark.org/review/921
Reviewed-by: Hadriel Kaplan <hadrielk@yahoo.com>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>