First set of IIO fixes for the 4.4 cycle.

This set does not include those for issues introduced during the merge
window.  Fixes of those will follow in a future series.

* ad5064
  - Make sure the local i2c_write returns 0 on success rather than the
    number of bytes transfered.  Otherwise we report an error on all writes.
  - Fix a shift for ad5629 and ad5669 which gives incorrect DAC output on
    these parts.
* ad7793
  - The product ID on the datasheet is wrong.  Fix it in the driver.
* IIO_DUMMY_EVGEN
  - select IRQ_WORK as a dependency.
* lpc32xx
  - make sure clock is prepared before enabling.
* si7020
  - data byte order was reversed. Fix it.
* vf610
  - Internal temperature calculation was wrong if a different
    reference voltage was used.  Now use a linear interpolation
    function to make it work over the full range.
  - Fix a division by zero in the case of a device tree property
    not being present (same issue two fixes).
* xilinx XADC
  - VREFN scale was wrong - fix it.
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