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: NHenk Vergonet <henk.vergonet@gmail.com>
Reviewed-By: NPali Rohár <pali.rohar@gmail.com>
Reviewed-By: NMario Limonciello <mario.limonciello@dell.com>
Signed-off-by: NAndy Shevchenko <andriy.shevchenko@linux.intel.com>

drivers/platform/x86/dell-wmi.c

@@ -233,7 +233,7 @@ static const u16 bios_to_linux_keycode[256] = {
 	[18]	= KEY_PROG1,
 	[19]	= KEY_BRIGHTNESSDOWN,
 	[20]	= KEY_BRIGHTNESSUP,
-	[21]	= KEY_UNKNOWN,
+	[21]	= KEY_BRIGHTNESS_AUTO,
 	[22]	= KEY_KBDILLUMTOGGLE,
 	[23]	= KEY_UNKNOWN,
 	[24]	= KEY_SWITCHVIDEOMODE,