IB/mlx4: Unregister IB device prior to CLOSE PORT command

According to the ConnectX programmer's reference manual, all
operations should be stopped, all QPs should be torn down and all WQEs
flushed before the CLOSE_PORT command is invoked.  In some cases
reversing the order of operations (as implemented now) could cause
a loss of completions.

Signed-off-by: Yevgeny Petrilin <yevgenyp@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>