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 spi_read() Add a DMA-safe buffer and use it for spi_read() 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 and __be32 and change the conversion functions appropriately.