iio: adc: mp2629: fix wrong comparison of channel

Input voltage channel enum is compared against iio address instead
of the channel.

Fixes: 7abd9fb6

 ("iio: adc: mp2629: Add support for mp2629 ADC driver")
Signed-off-by: Saravanan Sekar <sravanhome@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20221029093000.45451-2-sravanhome@gmail.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>