ixgbe: Add function for obtaining FCoE TC based on FCoE user priority

In upcoming patches it will become increasingly common to need to determine
the FCoE traffic class in order to determine the correct queues for FCoE.
In order to make this easier I am adding a function for obtaining the FCoE
traffic class based on the user priority.

Cc: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>