IB/mlx5: Make function static

mlx5_query_mad_ifc_smp_attr_node_info() is internal to mad.c Hence, make
it static.

Link: https://lore.kernel.org/r/20210113121703.559778-5-leon@kernel.org


Signed-off-by: Parav Pandit <parav@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>