iio: backend: fix kernel-doc to avoid warnings and ensure consistency

iio: backend: fix kernel-doc to avoid warnings and ensure consistency

Fix multiple kernel-doc warnings and make the documentation style
consistent in drivers/iio/industrialio-backend.c.

Changes include:
 - Add missing @chan parameter description in
   iio_backend_oversampling_ratio_set().
 - Add missing RETURNS section in iio_backend_get_priv().
 - Replace Return: with “RETURNS:” across the file for consistency.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202506292344.HLJbrrgR-lkp@intel.com
Suggested-by: Andy Shevchenko <andy@kernel.org>
Suggested-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Kriish Sharma <kriish.sharma2006@gmail.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>