net/mlx5: Fix root flow table update

When we destroy the last flow table we need to update
the root_ft to NULL.

It fixes an issue for when the last flow table is destroyed
and recreated again, root_ft pointer will not be updated,
as a result traffic will be dropped.

Fixes: 2cc43b49 ('net/mlx5_core: Managing root flow table')
Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>