iio: core: Fix the kernel doc regarding the currentmode iio_dev entry

This is an internal variable, which should be accessed in a very
sporadic way and in no case changed by any device driver.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Link: https://lore.kernel.org/r/20211215151344.163036-2-miquel.raynal@bootlin.com


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