hwmon: (iio_hwmon) use dev_err_probe

Instead of just returning an error code, add an error message as well.
While at it, simplify the code and use a common return path.
Upon deferral this also nicely lists the following message in
/sys/kernel/debug/devices_deferred:
adc     iio_hwmon: Failed to get channels
Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Link: https://lore.kernel.org/r/20230131103359.625081-1-alexander.stein@ew.tq-group.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/iio_hwmon.c

@@ -77,9 +77,11 @@ static int iio_hwmon_probe(struct platform_device *pdev)
 
 	channels = devm_iio_channel_get_all(dev);
 	if (IS_ERR(channels)) {
-		if (PTR_ERR(channels) == -ENODEV)
-			return -EPROBE_DEFER;
-		return PTR_ERR(channels);
+		ret = PTR_ERR(channels);
+		if (ret == -ENODEV)
+			ret = -EPROBE_DEFER;
+		return dev_err_probe(dev, ret,
+				     "Failed to get channels\n");
 	}
 
 	st = devm_kzalloc(dev, sizeof(*st), GFP_KERNEL);