mlxsw: spectrum_buffers: Introduce shared buffer ops

The size of the internal buffer is currently calculated in the SPAN module.
Logically it belongs to the spectrum_buffers module, where it should be
moved. However, that being a chip-specific operation, it needs dynamic
dispatch. There currently is a chip-specific structure for description of
shared buffer values, struct mlxsw_sp_sb_vals. However placing ops into
this structure would be confusing. Therefore introduce a new per-chip
structure, currently empty, and initialize the ops pointer as appropriate.

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>