ixgbe: X540 devices RX PFC frames pause traffic even if disabled

ixgbe: X540 devices RX PFC frames pause traffic even if disabled

Receiving PFC (priority flow control) frames while the feature
is off should not pause the traffic class. On the X540 devices
the traffic class react to frames if it was previously enabled
because the field is incorrectly cleared.

(cherry picked from commit 6b8456c0199c4dd35bb7a04ff299ab925699e390)
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>