Third set of fixes for IIO in the 4.7 cycle.

A couple of really old bugs and the results of Mark taking a close look at
some nasty regulator handling.

* ad7266
  - Fix broken regulator handling that won't play well with dummy regulators.
  - Correctly handle and optional regulator.
  - Fix probe deferral for the vref regulator.
* kxsd9
  - Fix a wrong error check that leads to an inability to write or read
  the scale.
* sca3000
  - Fix a wrong error check that leads to an inability to read back the
  sampling frequency.
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