ssa.c: In ssa_downstream_close_ssa_conn, minor code refactoring

Signed-off-by: Hal Rosenstock <hal@mellanox.com>
ssa.c: Check for existing SMDB before sending DB update notification

Signed-off-by: Ilya Nelkenbaum <ilyan@mellanox.com>
Signed-off-by: Hal Rosenstock <hal@mellanox.com>