platform: mellanox: mlx-platform: Modify graceful shutdown callback and power down mask

Use kernel_power_off() instead of kernel_halt() to pass through
machine_power_off() -> pm_power_off(), otherwise axillary power does
not go off.

Change "power down" bitmask.

Fixes: dd635e33

 ("platform: mellanox: Introduce support of new Nvidia L1 switch")
Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Reviewed-by: Michael Shych <michaelsh@nvidia.com>
Link: https://lore.kernel.org/r/20230813083735.39090-4-vadimp@nvidia.com
Signed-off-by: Hans de Goede <hdegoede@redhat.com>