ixgbe: use known user priority for FCoE when DCB is enabled

Store the user priority for FCoE and use it directly for outgoing
FCoE traffic when DCB is enabled.

Signed-off-by: Yi Zou <yi.zou@intel.com>
Acked-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>