ixgbe: When in DCB mode with PFC enabled, show LFC is disabled

Ethtool should report that link flow control is disabled when in priority
flow control mode.

Signed-off-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>