mlxsw: spectrum_buffers: Keep mlxsw_sp_sb_cm in sb_vals

The SBCM register configures shared buffer quota according to
port-priority resp. port-TC. The default configuration depends on the
chip type. Therefore keep the tables and their lengths 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>