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 <[e-mail]>
Closes: https://lore.kernel.org/oe-kbuild-all/[e-mail]
Suggested-by: Andy Shevchenko <[e-mail]>
Suggested-by: David Lechner <[e-mail]>
Signed-off-by: Kriish Sharma <[e-mail]>
Reviewed-by: Nuno Sá <[e-mail]>
Reviewed-by: Andy Shevchenko <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/industrialio-backend.c

···
  * interface/data bus. Hence, the backend device needs to be aware of it so
  * data can be correctly transferred.
  *
- * Return:
+ * RETURNS:
  * 0 on success, negative error number on failure.
  */
 int iio_backend_data_size_set(struct iio_backend *back, unsigned int size)
···
 /**
  * iio_backend_oversampling_ratio_set - set the oversampling ratio
  * @back: Backend device
+ * @chan: Channel number
  * @ratio: The oversampling ratio - value 1 corresponds to no oversampling.
  *
- * Return:
+ * RETURNS:
  * 0 on success, negative error number on failure.
  */
 int iio_backend_oversampling_ratio_set(struct iio_backend *back,
···
 /**
  * iio_backend_get_priv - Get driver private data
  * @back: Backend device
+ *
+ * RETURNS:
+ * Pointer to the driver private data associated with the backend.
  */
 void *iio_backend_get_priv(const struct iio_backend *back)
 {