ixgbe: fix ixgbe_tx_is_paused logic

The TFCS bits show the current XON state.  Meaning that the
device is paused if these bits are 0.  This fixes the logic
to work as it was intended.

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>