CVE-2026-72480: iio: adc: xilinx-ams: fix out-of-bounds channel lookup in event handling

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

iio: adc: xilinx-ams: fix out-of-bounds channel lookup in event handling

amseventtochannel() may return a pointer past the end of dev->channels when no matching scanindex is found. This can lead to invalid memory access in amshandleevent().

Add a bounds check in amseventtochannel() and return NULL when no channel is found. Also guard the caller to safely handle this case.