msi-wmi: depend on backlight and fix corner-cases problems

Now depends on BACKLIGHT_CLASS_DEVICE.
Driver will return an error if it can't get actual backlight value
Fix remapping of brightness keys when backlight is not controlled by ACPI.

Signed-off-by: Anisse Astier <anisse@astier.eu>
Signed-off-by: Len Brown <len.brown@intel.com>
diff --git a/drivers/platform/x86/Kconfig b/drivers/platform/x86/Kconfig
index 1f82d6d..a006dec 100644
--- a/drivers/platform/x86/Kconfig
+++ b/drivers/platform/x86/Kconfig
@@ -369,6 +369,7 @@
 	tristate "MSI WMI extras"
 	depends on ACPI_WMI
 	depends on INPUT
+	depends on BACKLIGHT_CLASS_DEVICE
 	select INPUT_SPARSEKMAP
 	help
 	 Say Y here if you want to support WMI-based hotkeys on MSI laptops.