platform/x86: dell-wmi: Ignore keyboard backlight change KBD_LED_ON_TOKEN

There's a wmi event generated by dell-wmi when pressing keyboard backlight
toggle key:
[1224203.948894] dell_wmi: Unknown key with type 0x0011 and code 0x01e2 pressed

This event is for notification purposes, let's ignore it.

Signed-off-by: Rhys Kidd <rhyskidd@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>