iio: inkern: Check error explicitly in iio_channel_read_max()

The current implementation returns the error code as part of the
default switch case.
This can lead to returning an incorrect positive value in case of
iio_avail_type enum entries evolution.

In order to avoid this case, be more strict in error checking.
Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Link: https://lore.kernel.org/r/20230623085830.749991-4-herve.codina@bootlin.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/iio/inkern.c

@@ -858,6 +858,9 @@ static int iio_channel_read_max(struct iio_channel *chan,
 		val2 = &unused;
 
 	ret = iio_channel_read_avail(chan, &vals, type, &length, info);
+	if (ret < 0)
+		return ret;
+
 	switch (ret) {
 	case IIO_AVAIL_RANGE:
 		switch (*type) {
@@ -888,7 +891,7 @@ static int iio_channel_read_max(struct iio_channel *chan,
 		return 0;
 
 	default:
-		return ret;
+		return -EINVAL;
 	}
 }