platform/wmi: Make wmi_bus_class const

platform/wmi: Make wmi_bus_class const

The functions class_register()/_unregister() and device_create()
both support taking a const pointer to the class struct. Use this
to mark wmi_bus_class as const so that it can be placed into
read-only memory for better security.

Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Link: https://patch.msgid.link/20260610203453.816254-8-W_Armin@gmx.de
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>