platform/x86: wmi: add missing struct parameter description

Add a description for the context parameter in the struct wmi_device_id.

Reported-by: kbuild test robot <lkp@intel.com>
Fixes: a48e2338 ("platform/x86: wmi: add context pointer field to struct wmi_device_id")
Signed-off-by: Mattias Jacobsson <2pi@mok.nu>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>