mlxsw: spectrum_buffer: Reduce scope of shared buffer struct

The shared buffer structure ('mlxsw_sp_sb') doesn't need to be
accessible to anyone, but the shared buffer code located at
spectrum_buffers.c

Make this apparent and reduce its scope by defining it there.

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