iio: inkern: Remove the 'unused' variable usage in iio_channel_read_max()

iio: inkern: Remove the 'unused' variable usage in iio_channel_read_max()

The code uses a local variable to initialize a null pointer in order to
avoid accessing this null pointer later on.

Simply removed the 'unused' variable and check for the null pointer just
before accessing it.

Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Link: https://lore.kernel.org/r/20230623085830.749991-6-herve.codina@bootlin.com
Signed-off-by: Mark Brown <broonie@kernel.org>