ixgbe: always enable vlan strip/insert when DCB is enabled

when DCB mode is on, we want the HW VLAN stripping to be always enabled.

Signed-off-by: Yi Zou <yi.zou@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>