net/mlx4: Move the tunnel steering helper function to mlx4_core

Move the function which we use to set VXLAN DMFS (flow-steering) rules
from mlx4_en to mlx4_core. This refactoring will allow the mlx4_ib driver
to call the helper for the use case of user-space RAW Ethernet QPs, such
that they can serve VXLAN traffic too.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>