iio: ltc2983: fail probe if no channels are given

If there are no channels defined in the devicetree, there's no point in
probing the device. We were actually requesting a zero sized 'kmalloc'
array but since we were not touching the ZERO_SIZE_PTR afterwards,
nothing bad was actually happening. Hence this is not really a fix but
rather an improvement.
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20210825084149.11587-2-nuno.sa@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/temperature/ltc2983.c

@@ -1275,6 +1275,11 @@ static int ltc2983_parse_dt(struct ltc2983_data *st)
 			     &st->filter_notch_freq);
 
 	st->num_channels = of_get_available_child_count(dev->of_node);
+	if (!st->num_channels) {
+		dev_err(&st->spi->dev, "At least one channel must be given!");
+		return -EINVAL;
+	}
+
 	st->sensors = devm_kcalloc(dev, st->num_channels, sizeof(*st->sensors),
 				   GFP_KERNEL);
 	if (!st->sensors)