net/mlx5: Add API to set the namespace steering mode

Add API to set the flow steering root namesapce mode.
Setting new mode should be called before any steering operation
is executed on the namespace.
This API is going to be used by steering users such switchdev.

Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>