net/mlx5: Add helper to initialize a flow steering actions struct instance

There are bunch of places in the code where the intermediate struct
that keeps the elements related to flow actions is initialized with
the same default values. Put that into a small DECLARE type helper.

This patch doesn't change any functionality.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Reviewed-by: Hadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>