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

No reason for this to be exposed to the drivers, so lets move it to the
opaque structure.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Link: https://lore.kernel.org/r/20210426174911.397061-8-jic23@kernel.org