dma: Have dma_buf_rw() take a void pointer

DMA operations are run on any kind of buffer, not arrays of
uint8_t. Convert dma_buf_rw() to take a void pointer argument
to save us pointless casts to uint8_t *.

Reviewed-by: Klaus Jensen <k.jensen@samsung.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20211223115554.3155328-8-philmd@redhat.com>