HID: wacom: generic: Ignore HID_DG_BATTERYSTRENTH == 0

AES sensors use the value 0 to indicate "not available" rather than
"completely dead". Such values are often sent for dozens of reports
while the pen is being brought into proximity and can cause userspace
to get the wrong impression about the actual battery state.

Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Reviewed-by: Ping Cheng <ping.cheng@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>