RTIO: default to the fallback function when driver doesnt support RTIO

[VynDragon]

Description
When using a driver that uses RTIO (for example sensor async api provider), it will not default to the i2c_iodev_submit_fallback function if it has not been implemented by the driver.

Solution
Implement automatic default fallback.

[ubieda]

Thanks for capturing the issue.

IIRC - There was a reason why the auto-fallback was not implemented for bus-drivers. I think the run-time check was something we wanted to stay away from (it would apply both to those having it). However, I see for I2C and I3C we do have a run-time check that resolves in an error:

zephyr/include/zephyr/drivers/i2c.h

Lines 1053 to 1064 in e1a0350

	static inline void i2c_iodev_submit(struct rtio_iodev_sqe *iodev_sqe)
	{
	const struct i2c_dt_spec *dt_spec = (const struct i2c_dt_spec *)iodev_sqe->sqe.iodev->data;
	const struct device *dev = dt_spec->bus;
	const struct i2c_driver_api *api = (const struct i2c_driver_api *)dev->api;
	if (api->iodev_submit == NULL) {
	rtio_iodev_sqe_err(iodev_sqe, -ENOSYS);
	return;
	}
	api->iodev_submit(dt_spec->bus, iodev_sqe);
	}

zephyr/include/zephyr/drivers/i3c.h

Lines 2481 to 2492 in e1a0350

	static inline void i3c_iodev_submit(struct rtio_iodev_sqe *iodev_sqe)
	{
	const struct i3c_iodev_data *data =
	(const struct i3c_iodev_data *)iodev_sqe->sqe.iodev->data;
	const struct i3c_driver_api *api = (const struct i3c_driver_api *)data->bus->api;
	if (api->iodev_submit == NULL) {
	rtio_iodev_sqe_err(iodev_sqe, -ENOSYS);
	return;
	}
	api->iodev_submit(data->bus, iodev_sqe);
	}

This is not consistent with SPI:

zephyr/include/zephyr/drivers/spi.h

Lines 1234 to 1241 in e1a0350

	static inline void spi_iodev_submit(struct rtio_iodev_sqe *iodev_sqe)
	{
	const struct spi_dt_spec *dt_spec = (const struct spi_dt_spec *)iodev_sqe->sqe.iodev->data;
	const struct device *dev = dt_spec->bus;
	const struct spi_driver_api *api = (const struct spi_driver_api *)dev->api;
	api->iodev_submit(dt_spec->bus, iodev_sqe);
	}

To move forward, we should standardize this pattern, by either:

1. Auto-fallback should the submit API is not implemented, or
2. Throw a compile-time error indicating that a particular bus-driver does not implement it, while the corresponding Kconfig flags are enabled (e.g: CONFIG_I2C_RTIO and CONFIG_SENSOR_ASYNC_API).

I would rather prefer No. 2, but TBD how that would be implemented.

[ubieda]

@teburd @yperess feel free to chime in here

[teburd]

Compile time error is the right option here imho