iio: lmp91000: Set parent device

Initialize the parent of the IIO device to the device that registered it.
This makes sure that the IIO device appears the right level in the device
hierarchy.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Reviewed-by: Matt Ranostay <matt.ranostay@konsulko.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>