iio:core: Tidy up kernel-doc.

One comment isn't kernel-doc at all, but starts with /** and another
is simply missing a parameter that was introduced recently.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20200913132115.800131-4-jic23@kernel.org