iio: dummy: Use automatic lock and direct mode cleanup.

Given we now have iio_device_claim_direct_scoped() to perform automatic
releasing of direct mode at exit from the scope that follows it, this can
be used in conjunction with guard(mutex) etc remove a lot of special case
handling.

Note that in this particular example code, there is no real reason you can't
read channels via sysfs at the same time as filling the software buffer.
To make it look more like a real driver constrain raw and processed
channel reads from occurring whilst the buffer is in use.

Reviewed-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Nuno Sa <nuno.a@analog.com>
Link: https://lore.kernel.org/r/20240128150537.44592-3-jic23@kernel.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>