platform/x86: dell-wmi: Ignore new keyboard backlight change event

There's a new wmi event generated by dell-wmi when pressing keyboard
backlight hotkey:
[ 3285.474172] dell_wmi: Unknown key with type 0x0010 and code 0x003f pressed

This event is for notification purpose, let's ignore it. The keyboard
backlight hotkey uses another event so it still works without event
0x3f.

Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Reviewed-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>