mlxsw: spectrum_qdisc: Disable port buffer autoresize with qdiscs

There are two interfaces to configure ETS: qdiscs and DCB. Historically,
DCB ETS configuration was projected to ingress as well, and configured port
buffers. Qdisc was not.

Keep qdiscs behaving this way, and if an offloaded qdisc is configured on a
port, move this port's headroom to a manual mode, thus allowing
configuration of port buffers through dcbnl_setbuffer.

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