iio: core: move @flags from struct iio_dev to struct iio_dev_opaque

No reason any driver should ever need access to this field, so hide it.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Link: https://lore.kernel.org/r/20210426174911.397061-9-jic23@kernel.org