ixgbe: dcb, set DPF bit when PFC is enabled

Set the DPF bit when PFC is enabled.  This will discard
PFC frames so they do not get passed up the stack.

The DPF bit is set for flow control, but not priority
flow control this brings pfc inline with fc.

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>