driver: sensor: adxl367: Fix for SPI

Fix for SPI communication when RTIO is used in SPI driver. When in
adxl367_bus_access, const struct spi_buf buf[3] is used,
spi_rtio_copy function won't set correct buffers and length
for buf[2] and that will cause exception when that buffer is
processed.

Signed-off-by: Vladislav Pejic <vladislav.pejic@orioninc.com>