mlxsw: spectrum_buffers: Allow pools of infinite size

The MC pool should have an infinite size (i.e. no quota).

To that end, add infi_size to the SBPR register and extend
mlxsw_reg_sbpr_pack(). Also add MLXSW_SP_SB_INFI to denote
buffers that should have an infinite size.

Change mlxsw_sp_sb_pr_write() to take as parameter byte size,
instead of cell size, and add the special handling of infinite
buffers. Report pools with infinite size as if they actually
take the full 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>