ixgbe: implement DCB ops dcb_ieee_del()

Implement DCB ops dcb_ieee_del() and set FCoE to the default
priority when no priority exists.

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>