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.com


Signed-off-by: Mark Brown <broonie@kernel.org>