platform_data/mlxreg: Update module license

Update license to SPDX-License.

Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20200923172053.26296-4-vadimp@nvidia.com


Signed-off-by: Hans de Goede <hdegoede@redhat.com>