mlxsw: spectrum_buffers: Use defines for pool indices

The pool indices are currently hard coded throughout the code, which
makes the code hard to follow and extend.

Overcome this by using defines for the pool indices.

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