net/mlx5: Bridge, set debugfs access right to root-only

As suggested during code review set the access rights for bridge 'fdb'
debugfs file to root-only.

Fixes: 791eb782 ("net/mlx5: Bridge, expose FDB state via debugfs")
Reported-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/netdev/20230619120515.5045132a@kernel.org/


Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>