iio: adc: ad7173: fix channels index for syscalib_mode

In the Linux kernel, the following vulnerability has been resolved:

iio: adc: ad7173: fix channels index for syscalib_mode

Fix the index used to look up the channel when accessing the syscalib_mode attribute. The address field is a 0-based index (same as scan_index) that it used to access the channel in the ad7173_channels array throughout the driver. The channels field, on the other hand, may not match the address field depending on the channel configuration specified in the device tree and could result in an out-of-bounds access.