net/mlx5: Fix flow steering NIC capabilities check

Flow steering infrastructure is currently used only on link layer
ethernet, therefore the driver should initialize the flow steering
when the device link layer is ethernet.

In addition, add missing capability check before initializing the
namespace of NIC RX flow tables.

Fixes: 25302363 ('net/mlx5_core: Flow steering tree initialization')
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>