Staging: iio: adt7316: Fix i2c data reading, set the data field

adt7316_i2c_read function nowhere sets the data field.
It is necessary to have an appropriate value for it.
Hence, assign the value stored in 'ret' variable to data field.

This is an ancient bug, and as no one seems to have noticed,
probably no sense in applying it to stable.

Signed-off-by: Shreeya Patel <shreeya.patel23498@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>