[nios] VCTCXO Trim DAC Readback is implemented incorrectly

[jynik]

Here in the NIOS code we currently perform an SPI transaction in the form:

[ Command | Read Data ]

However, the DAC actually requires two separate transactions to retrieve the register data:

[ Command | Dummy Data] [ Command | Read data ]

Fortunately, this defect does not cause users to see invalid values. When clocking in "Dummy Data," the device is clocking out data from the previous command. This will either be previous read data or the data we were writing to the VCTCXO. Given the manner in which we use the device, both should be "correct."

Thus, this issue is a lower-priority "to do" item.

Reference: