ssa.c: Add IBV_EVENT_SM_CHANGE event to device's event processing routines

The event is generated when there is a new SM in the subnet.
WARNING. The event is not generated by Mellanox drivers.
The event is handled for debug purposes.

Signed-off-by: Sasha Kotchubievsky <sashakot@mellanox.com>
Signed-off-by: Hal Rosenstock <hal@mellanox.com>