HID: steelseries: export charging state for the SteelSeries Arctis 9 headset

The Arctis 9 headset provides the information if
the power cable is plugged in and charging via the battery report.
This information can be exported.

Signed-off-by: Christian Mayer <git@mayer-bgk.de>
Reviewed-by: Bastien Nocera <hadess@hadess.net>
Tested-by: Bastien Nocera <hadess@hadess.net>
Signed-off-by: Jiri Kosina <jkosina@suse.com>