net/mlx5: rename confusing object in probe function

net/mlx5: rename confusing object in probe function

There are several attribute objects in this function:

- IB device attributes (struct ibv_device_attr_ex device_attr).
- Direct Verbs attributes (struct mlx5dv_context attrs_out).
- Port attributes (struct ibv_port_attr).
- IB device attributes again (struct ibv_device_attr_ex device_attr_ex).

"attrs_out" is both odd and initialized using a nonstandard syntax. Rename
it "dv_attr" for consistency.

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Reviewed-by: Xueming Li <xuemingl@mellanox.com>