Third IIO new drivers and cleanups series for 3.15.

New driver
* Xilinx XADC driver -  This has been ready for a while but was awaiting
  a device tree ack (or as it turns out 3+ weeks).

Cleanup
* Drop some unreachable code from mag3110 highlighted by smatch.

Fix
* vf610 - introduced this cycle - put a possible negative error code
  into an unsigned long. Another smatch find - this one promoted by
  guilt that Dan was busy fixing all our messups.
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