HID: input: throttle battery uevents

The power_supply subsystem tends to emit uevent every time
power_supply_changed() is called, so we should call this API only when battery
strength reported by the device is actually different from the previous
readings, otherwise we'll drown the system in uevents.

Fixes: 581c4484

 ("HID: input: map digitizer battery usage")
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>