iio: dummy: iio_simple_dummy: Demote unworthy kerneldocs and correct wrong parameter in docs

File header comments are not good candidates for kerneldoc.  Neither
are generic comment blocks.

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

 drivers/iio/dummy/iio_simple_dummy.c:26: warning: cannot understand function prototype: 'const struct config_item_type iio_dummy_type = '
 drivers/iio/dummy/iio_simple_dummy.c:564: warning: Function parameter or member 'name' not described in 'iio_dummy_probe'
 drivers/iio/dummy/iio_simple_dummy.c:564: warning: Excess function parameter 'index' description in 'iio_dummy_probe'
 drivers/iio/dummy/iio_simple_dummy.c:700: warning: cannot understand function prototype: 'const struct iio_sw_device_ops iio_dummy_device_ops = '

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