iio: adc: mp2629: fix potential array out of bound access

Add sentinel at end of maps to avoid potential array out of
bound access in iio core.

Fixes: 7abd9fb6 ("iio: adc: mp2629: Add support for mp2629 ADC driver")
Signed-off-by: Saravanan Sekar <sravanhome@gmail.com>
Link: https://lore.kernel.org/r/20221029093000.45451-4-sravanhome@gmail.com


Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>