HID: hid-asus: Add BT keyboard dock battery monitoring support

Add battery monitoring support for Transbook T100CHI/T90CHI's Bluetooth
keyboard dock.  They report rest battery level and charging status.

Signed-off-by: NOGUCHI Hiroshi <drvlabo@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>