mlxsw: spectrum_cnt: Add entry_size_res_id for each subpool and use it to query entry size

Add new field to subpool struct that would indicate which
resource id should be used to query the entry size for
the subpool from the device.

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>