mlxsw: spectrum_buffers: Keep pool descriptors in mlxsw_sp_sb_vals

Keep the table of pool descriptors and its length in struct
mlxsw_sp_sb_vals so that it can be specialized per chip type. Redirect
all users from the global definitions to the mlxsw_sp_sb fields.

Give mlxsw_sp_pool_count() an extra mlxsw_sp parameter so that it can
access the descriptor table.

Drop the now unnecessary MLXSW_SP_SB_POOL_DESS_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>