ixgbe: add write flush when configuring CS4223/7

ixgbe: add write flush when configuring CS4223/7

Make sure the writes are processed immediately. Without the flush it
is possible for operations on one port to spill over the other as the
resource is shared.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Orabug: 26242766
(cherry picked from commit 410a494902777c11f95031d9ed757d7f8f09c5c6)
Signed-off-by: Jack Vogel <jack.vogel@oracle.com>
Reviewed-by: Ethan Zhao <ethan.zhao@oracle.com>