iio: dac: ad5592r: remove usage of iio_priv_to_dev() helper

This was partially removed when the mlock cleanup was done. Only one more
call is left in the ad5592r_alloc_channels() function.
This one is simple. We just need to pass the iio_dev object and get the
state via iio_priv().

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>