staging:iio:ade7758_spi_read_burst: Pass IIO device directly

When calling ade7758_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>