The kerneldoc description for the trig_readonly field of struct iio_dev
lacked a colon, leading to this doc build warning:

  ./include/linux/iio/iio.h:603: warning: No description found for parameter 'trig_readonly'

A similar issue for iio_trigger_set_immutable() in trigger.h yielded:

  ./include/linux/iio/trigger.h:151: warning: No description found for parameter 'indio_dev'
  ./include/linux/iio/trigger.h:151: warning: No description found for parameter 'trig'

Fix the formatting and silence the warnings.
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>