demo: Provide dev_clear()

demo: Provide dev_clear()

Devices for the demo driver do have additional data attached to it that
needs to be freed when the device is freed. While the driver gets it right
for the cleanup() callback it does not implement a dev_clear() callback, so
the default dev_clear() implementation is used which will not free the
additional data. This will cause memory leaks when sr_dev_clear() is called
for this driver.

To fix this provide a dev_clear() implementation that frees the additional
data.

Signed-off-by: Lars-Peter Clausen <redacted>