mlxsw: spectrum_buffers: Tweak SBMM configuration

The SBMM register configures shared buffer allocation and settings for
MC packets according to switch priority. The recommended values are no
reserved buffer and alpha of 1/4, which corresponds to buf_max of 6.
Update mlxsw_sp_sb_mms accordingly.

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