iio: dummy: iio_simple_dummy_buffer: Demote file header and correct misspelling

File headers are not good candidates for kerneldoc.

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

 drivers/iio/dummy/iio_simple_dummy_buffer.c:27: warning: cannot understand function prototype: 'const s16 fakedata[] = '
 drivers/iio/dummy/iio_simple_dummy_buffer.c:185: warning: Function parameter or member 'indio_dev' not described in 'iio_simple_dummy_unconfigure_buffer'
 drivers/iio/dummy/iio_simple_dummy_buffer.c:185: warning: Excess function parameter 'indo_dev' description in 'iio_simple_dummy_unconfigure_buffer'

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