iio: avoid shadowing of variable name in to_iio_dev_opaque()

indio_dev was both the macro input parameter and the field name
in this macro.  That causes trouble if the instance of
struct iio_dev passed in is not called indio_dev.

Whilst a fix of sorts, no need to backport as it seems we never
hit this previously due to some very consistent naming in IIO.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Link: https://lore.kernel.org/r/20210426174911.397061-3-jic23@kernel.org