net/mlx5: Add modify FTE helper function

Add modify FTE helper function and use it when deleting a rule, as a
pre-step towards consolidated FTE modification, with no functional
change.

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