mlxsw: spectrum_buffers: Allow an infinite maximum for per-TC pool limit

The SBCM register configures the shared buffer configuration according
to port and TC. So far all pools have had a dynamic size, where the
infinite size is easy to express by using max_buff of 0xff. However the
MC pool should be configured with static size, and the infinite size
thus needs to be set using the field SBCM.infi_max.

Therefore add the field infi_max to the SBCM register and to
mlxsw_reg_sbcm_pack(). Extend mlxsw_sp_sb_cm_write() to handle infinite
sizes as well. Report infinite pool limits as if the limit actually were
the total shared buffer size.

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>