staging:iio:adis16260_read_ring_data: Pass IIO device directly

When calling adis16260_read_ring_data we pass the device struct of embedded in
the IIO device only to look up the IIO device from the device struct again right
away. This patch changes the code to pass the IIO device directly.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>