staging:iio:adis16400_ring: Pass IIO device directly

When calling adis16350_spi_read_all and adis16400_spi_read_burst 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>