IB/mlx4: Change flush logic so it adheres to the variable name

Change and simplify the code to match the variable name. This commit
improves readability but makes no functional change to the code.

Signed-off-by: Håkon Bugge <haakon.bugge@oracle.com>
Suggested-by: Wengang Wang <wen.gang.wang@oracle.com>
Reviewed-by: Wengang Wang <wen.gang.wang@oracle.com>
Reviewed-by: Knut Omang <knut.omang@oracle.com>
Acked-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>