net/mlx5: Properly handle a vport destination when setting FTE

When creating FTE, properly distinguish between destination being vport
or tir. The previous code just worked accidentally b/c of both dest being
in the same offset within a union.

Signed-off-by: Shahar Klein <shahark@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>