ixgbe: PFC not cleared on X540 devices

X540 devices do not clear PFC before sets. This results in
the device possibly responding to PFC frames that the user
has disabled. Although it would also be wrong for the peer
to be transmitting these frames. Now we clear the register
before set.

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>