iio: core: fix/re-introduce back parent assignment

This was introduced initially via commit 78289b4a ("iio: core: pass
parent device as parameter during allocation"), but was accidentally
removed via commit 6d4ebd56 ("iio: core: wrap IIO device into an
iio_dev_opaque object").

This looks like a rebase gone wrong, and ends up breaking devicetree
bindings of IIO clients.

This change adds back the parent assignment.

Fixes 6d4ebd56

: ("iio: core: wrap IIO device into an iio_dev_opaque object")
Reported-by: Dmitry Baryshkov <dbaryshkov@gmail.com>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Tested-by: Dmitry Baryshkov <dbaryshkov@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>