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: NIdo Schimmel <idosch@mellanox.com>
Signed-off-by: NJiri Pirko <jiri@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>