net/mlx5: Expose FW version over defined keyword

To be aligned with the rest of the drivers, expose FW version under "fw"
keyword in devlink dev info, in addition to the existing "fw.version",
which is currently Mellanox-specific.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>