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

Continuing move to hide internal elements from drivers, move this structure
element over.  It's only accessed from iio core files so this one was
straight forward and no accessor functions are needed.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Link: https://lore.kernel.org/r/20210426174911.397061-4-jic23@kernel.org