platform/x86: wmi: declare device_type structure as constant

The only usage of device_type structure is getting stored as
a reference in the type field of device structure. This type
field is declared const. Therefore, the device_type structure
can never be modified and can be declared as const.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>