HID: Ignore battery for Elan touchscreen on Asus TP420IA

This device has a touchscreen thats report a battery even if it doesn't
have one.
Ask Linux to ignore the battery so it will not always report it as low.

[jkosina@suse.cz: fix whitespace damage]
Signed-off-by: Marco Rodolfi <marco.rodolfi@tuta.io>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>