power: generic-adc-battery: check for duplicate properties copied from iio channels

If an iio channel defines a basic property, there are duplicate entries
in /sys/class/power/*/uevent.

So add a check to avoid duplicates. Since all channels may be duplicates,
we have to modify the related error check.

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Cc: stable@vger.kernel.org
Fixes: e60fea79 ("power: battery: Generic battery driver using IIO")
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>