spi_rtio: Transaction OPs not working for Default handler

[ubieda]

Describe the bug

SPI RTIO default handler does not properly execute multi-transaction OPs (e.g: write + read in a single transfer).

Expected behavior

The expectation is to perform a single SPI transfer without de-asserting CS line.
What is currently happening is that the CS line is de-asserted at the end of each OP, resetting the state for the second or third operations.

Impact

In-tree Sensor drivers depend on this behavior, for instance:

• ICM42688:

  zephyr/drivers/sensor/tdk/icm42688/icm42688_rtio_stream.c

  Lines 155 to 158 in 1280f43

  	rtio_sqe_prep_tiny_write(write_fifo_addr, spi_iodev, RTIO_PRIO_NORM, &reg_addr, 1, NULL);
  	write_fifo_addr->flags = RTIO_SQE_TRANSACTION;
  	rtio_sqe_prep_read(read_fifo_data, spi_iodev, RTIO_PRIO_NORM, read_buf, read_len,
  	iodev_sqe);

• ADXL's:

  zephyr/drivers/sensor/adi/adxl345/adxl345_stream.c

  Lines 212 to 217 in 1280f43

  	rtio_sqe_prep_tiny_write(write_fifo_addr, data->iodev, RTIO_PRIO_NORM, &reg_addr,
  	1, NULL);
  	write_fifo_addr->flags = RTIO_SQE_TRANSACTION;
  	rtio_sqe_prep_read(read_fifo_data, data->iodev, RTIO_PRIO_NORM,
  	read_buf + data->fifo_total_bytes,
  	SAMPLE_SIZE, current_sqe);

• LSM6DSV:

  zephyr/drivers/sensor/st/lsm6dsv16x/lsm6dsv16x_rtio_stream.c

  Lines 358 to 361 in 1280f43

  	rtio_sqe_prep_tiny_write(write_fifo_dout_addr, iodev, RTIO_PRIO_NORM, &reg, 1, NULL);
  	write_fifo_dout_addr->flags = RTIO_SQE_TRANSACTION;
  	rtio_sqe_prep_read(read_fifo_dout_reg, iodev, RTIO_PRIO_NORM,
  	read_buf, buf_avail, lsm6dsv16x->streaming_sqe);

Additional context

A similar fix was already applied to i2c_rtio_default: #79890.

[ubieda]

Adding low as it impacts an experimental feature. Please override if anybody thinks otherwise.