If an external keyboard with a backlight gets unplugged then
up_kbd_backlight_event_io would constantly get called, burning 100% CPU.
To make things worse, up_kbd_backlight_event_io would also constantly post
DBUS events, causing gnome-shell to also become very unresponsive.
This commit fixes this by returning FALSE from up_kbd_backlight_event_io
on unplug.
While at it also fix calling up_kbd_backlight_emit_change with a negative
brightness value in other error scenarios. Specifically this fixes
calling up_kbd_backlight_emit_change with -1 on the initial
up_kbd_backlight_event_io call in which case up_kbd_backlight_brightness_read
will typically fail with ENODATA.
04ee9afb58