ixgbe: PFC not cleared on X540 devices

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.

(cherry picked from commit 634cdca5637475b74dbc7bd72208f5fdc5904d38)
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>
Signed-off-by: Joe Jin <joe.jin@oracle.com>