staging:iio:adis16260: Fix drvdata inconsistency

The SPI drvdata is set to adis16260_state struct in probe(), but assumed to be
the iio_dev struct in remove(). Fix this by setting it to the iio_dev in
probe().

The issue has been discovered using the following coccinelle patch:

// <smpl>
@r1@
type T;
T data;
expression dev;
@@
(
spi_set_drvdata(dev, (void *)data)
|
spi_set_drvdata(dev, data)
)

@r2@
type r1.T;
T data;
position p;
expression dev;
@@
data = spi_get_drvdata@p(dev)

@depends on r1@
position p != r2.p;
expression dev;
identifier data;
@@
*data = spi_get_drvdata@p(dev)
// </smpl>

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>