mlxsw: spectrum: Track buffer sizes in struct mlxsw_sp_hdroom

So far, port buffers were always autoconfigured. When dcbnl_setbuffer
callback is implemented, it will allow the user to change the buffer size
configuration by hand. The sizes therefore need to be a configuration
parameter, not always deduced, and therefore belong to struct
mlxsw_sp_hdroom, where the configuration routine should take them from.

Update mlxsw_sp_port_headroom_set() to update these sizes. Have the
function update the sizes even for the case that a given buffer is not
used.

Additionally, change the loop iteration end to DCBX_MAX_BUFFERS instead of
IEEE_8021QAZ_MAX_TCS. The value is the same, but the semantics differ.

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>