mlxsw: spectrum_buffers: silence uninitialized warning

Static checkers and runtime checkers such as KMSan will complain that
we do not initialize the last 6 bytes of "cb_priv".  The caller only
uses the first two bytes so it doesn't cause a runtime issue.  Still
worth fixing though.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>