iio: core: Fix issues and style of the comments

The `scripts/kernel-doc -v -none -Wall` reports several issues
with the kernel doc in IIO core C file. Update the comments
accordingly.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20230724110204.46285-5-andriy.shevchenko@linux.intel.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>