platform: mellanox: mlx-platform: Change register name

Register 0xd9 was repurposed on new systems. Change its name
to correctly reflect the new functionality.

Reviewed-by: Felix Radensky <fradensky@nvidia.com>
Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Link: https://lore.kernel.org/r/20250124172632.22437-4-vadimp@nvidia.com


Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>