mlxsw: spectrum_cnt: Move config validation along with resource register

Move the validation of subpools configuration, to avoid possible over
commitment to resource registration. Add WARN_ON to indicate bug
in the code.

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>