iio: dummy: iio_simple_dummy_events: Demote file header and supply descriptions for val2 params

File headers are not good candidates for kerneldoc.

Fixes the following W=1 kernel build warning(s):

 drivers/iio/dummy/iio_simple_dummy_events.c:21: warning: Incorrect use of kernel-doc format:  * iio_simple_dummy_read_event_config() - is event enabled?
 drivers/iio/dummy/iio_simple_dummy_events.c:34: warning: Function parameter or member 'indio_dev' not described in 'iio_simple_dummy_read_event_config'
 drivers/iio/dummy/iio_simple_dummy_events.c:34: warning: Function parameter or member 'chan' not described in 'iio_simple_dummy_read_event_config'
 drivers/iio/dummy/iio_simple_dummy_events.c:34: warning: Function parameter or member 'type' not described in 'iio_simple_dummy_read_event_config'
 drivers/iio/dummy/iio_simple_dummy_events.c:34: warning: Function parameter or member 'dir' not described in 'iio_simple_dummy_read_event_config'
 drivers/iio/dummy/iio_simple_dummy_events.c:123: warning: Function parameter or member 'val2' not described in 'iio_simple_dummy_read_event_value'
 drivers/iio/dummy/iio_simple_dummy_events.c:146: warning: Function parameter or member 'val2' not described in 'iio_simple_dummy_write_event_value'

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>