net/mlx5: DR, Some refactoring of miss address handling

In preparation for MATCH RANGE STE support, create a function
to set the miss address of an STE.

Signed-off-by: Yevgeny Kliteynik <kliteyn@nvidia.com>
Reviewed-by: Erez Shitrit <erezsh@nvidia.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>