platform/x86: dell-wmi: Set correct keycode for Fn + left arrow

Fn + left arrow hotkey combination is used for enabling/disabling automatic
display brightness based on integrated ALS sensor. For this purpose there
is standard Linux key KEY_BRIGHTNESS_AUTO so use it instead of KEY_UNKNOWN.

Tested on Dell Lattitude E6500.

Signed-off-by: Henk Vergonet <henk.vergonet@gmail.com>
Reviewed-By: Pali Rohár <pali.rohar@gmail.com>
Reviewed-By: Mario Limonciello <mario.limonciello@dell.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>