mlxsw: spectrum_cnt: Move sub_pools under per-instance pool struct

Currently, the global static array of subpools is used. Make it
per-instance as multiple instances of the mlxsw driver can have
different values.

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