mlxsw: core: Add mlxsw specific workqueue and use it for FDB notif. processing

Follow-up patch is going to need to use delayed work as well and
frequently. The FDB notification processing is already using that and
also quite frequently. It makes sense to create separate workqueue just
for mlxsw driver in this case and do not pollute system_wq.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>