Hans de Goede authored May 20, 2019 and Martin Pitt committed Jun 04, 2019

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.