net/mlx5: Return ready to use ASO WQE

There is no need in hiding returned ASO WQE type by providing void*,
use the real type instead. Do it together with zeroing that memory,
so ASO WQE will be ready to use immediately.

Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>