Second set of IIO fixes for the 4.7 cycle.

This includes one tracked regression (Arnd's patch for the ad7606).
The other two have I think always been broken.

* inv_mpu6050
  - Fix a use after free in the ACPI code.
* ad5933
  - The code for setting the cycles had a bug that meant it was simply wrong.
* ad7606_spi
  - Fix a regression that got introduced in a buggy cleanup of a sparse
    warning.
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