ixgbe: Cleanup FCOE and VLAN handling in xmit_frame_ring

This change is meant to further cleanup the transmit path by streamlining
some of the VLAN and FCOE/DCB tasks in the transmit path.  In addition it
adds code for support software VLANs in the event that they are used in
conjunction with DCB and/or FCOE.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>