CVE-2026-31768 iio: adc: ti-adc161s626: use DMA-safe memory for spi_read()

In the Linux kernel, the following vulnerability has been resolved: iio: adc: ti-adc161s626: use DMA-safe memory for spiread Add a DMA-safe buffer and use it for spiread instead of a stack memory. All SPI buffers must be DMA-safe. Since we only need up to 3 bytes, we just use a u8 instead of be16...