mlxsw: spectrum_cnt: Consolidate subpools initialization

Put all init operations related to subpools into
mlxsw_sp_counter_sub_pools_init().

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>