IB/cm: Flush workqueue when removing device

When a CM MAD is received, it is queued to a CM workqueue for
processing.  The queued work item references the port and device on
which the MAD was received.  If that device is removed from the system
before the work item can execute, the work item will reference freed
memory.

To fix this, flush the workqueue after unregistering to receive MAD,
and before the device is be freed.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>