mlxsw: spectrum_buffers: Keep mlxsw_sp_sb_mm in sb_vals

The SBMM register configures the shared buffer quota for MC packets
according to Switch-Priority. The default configuration depends on the
chip type. Therefore keep the table and length in struct
mlxsw_sp_sb_vals. Redirect the references from the global definitions to
the fields.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>