iio: adc: ti-ads8688: fix information leak in triggered buffer

In the Linux kernel, the following vulnerability has been resolved: iio: adc: ti-ads8688: fix information leak in triggered buffer The 'buffer' local array is used to push data to user space from a triggered buffer, but it does not set values for inactive channels, as it only uses iio_for_each_active_channel() to assign new values. Initialize the array to zero before using it to avoid pushing uninitialized information to userspace.