mlxsw: spectrum_buffers: Keep mlxsw_sp_sb_pms in mlxsw_sp_sb_vals

The SBPM register can be used to configure quotas for packets ingressing
from a certain pool to a certain port, and egressing from a certain pool
to a certain port. The default configuration depends on the chip type.
Therefore keep it in struct mlxsw_sp_sb_vals. Redirect the one reference
from the global array to the field.

Because the pool descriptor ID is implicit in the ordering of array
members, both this array and the pool descriptor array have the same
length. Therefore reuse mlxsw_sp_sb.pool_dess_len for the purpose of
determining the length of SBPM array.

Drop the now useless MLXSW_SP_SB_PMS_LEN.

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>