mlxsw: spectrum_dcb: Implement dcbnl_setbuffer / getbuffer

Add dcbnl_setbuffer, which bounces requests if a headroom is in DCB mode.
Implement dcbnl_getbuffer such that it can always be used to determine
port-buffer configuration, regardless of headroom mode.

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>