ixgbe: Cleanup unpacking code for DCB

This is meant to be a generic clean-up of the remaining functions for
unpacking data from the DCB structures. The only real changes are:
replaced the variable i with tc for functions that were looping through the
traffic classes, and added a pointer for tc_class instead of path since
that way we only need to pull the pointer once instead of once per loop.

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>