HID: Ignore battery for Elan touchscreen on ASUS UX550

Battery status is being reported for the Elan touchscreen on ASUS
UX550 laptops despite not having a batter. It always shows either 0 or
1%.

Signed-off-by: Seth Miller <miller.seth@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>