IB/mlx4: Avoid executing gid task when device is being removed

When device is being removed (e.g during VPI port link type change
from ETH to IB), tasks for gid table changes should not be executed.

Flush the current queue of tasks and block further tasks from entering the queue.

Signed-off-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>