mlxsw: spectrum_buffers: Add struct mlxsw_sp_hdroom

The port headroom handling is currently strewn across several modules and
tricky to follow: MTU, DCB PFC, DCB ETS and ethtool pause all influence the
settings, and then there is the completely separate initial configuraion in
spectrum_buffers. A following patch will implement the dcbnl_setbuffer
callback, which is going to further complicate the landscape.

In order to simplify work with port buffers, the following patches are
going to centralize all port-buffer handling in spectrum_buffers. As a
first step, introduce a (currently empty) struct mlxsw_sp_hdroom that will
keep the configuration parameters, and allocate and free it in appropriate
places.

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>