ixgbe: Enable another bit for flow control operation

The discard pause frames bit was not enabled, so flow control frames could
be passed up to the host.  Enabled DPF to keep the frames off the stack.

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>