mlxsw: spectrum_buffers: Remove assumption about pool order

The code currently assumes that ingress pools have lower indices than
egress pools. This makes it impossible to add more ingress pools
without breaking user configuration that relies on a certain pool index
to correspond to an egress pool.

Remove such assumptions from the code, so that more ingress pools could
be added by subsequent patches.

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>