IB/mlx4: Fix memory leak in do_slave_init

We create a number of work structs to be queued up to a workqueue, and
on completion of the workqueue handler, the workqueue handler frees the
allocated memory.  If, however, we don't queue the work struct because
the device is going down, then we need to free the memory ourselves.

Signed-off-by: Doug Ledford <dledford@redhat.com>