iio: adc: Use devm_krealloc_array

Now that it exists, use it instead of doing the multiplication and
checking for overflow manually.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: James Clark <james.clark@arm.com>
Link: https://lore.kernel.org/r/20230509094942.396150-4-james.clark@arm.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>