ixgbe: Simplify logic for getting traffic class from user priority

ixgbe: Simplify logic for getting traffic class from user priority

This patch is meant to help simplify the logic for getting traffic classes
from user priorities. To do this I am adding a function named
ixgbe_dcb_get_tc_from_up that will go through the traffic classes in
reverse order in order to determine which traffic class contains a bit for
a given user priority.

Adding a declaration for this new function to the header so that
we have a centralized means for sorting out traffic classes belonging to
features such as FCoE.

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>