First set of IIO fixes for the 4.19 cycle.

* ad9523
  - sysfs write should return the number of characters used or an error, not
    0 which could result in an infinite loop in userspace.
* lsm6dsx
  - Fix computation of length when updating the watermark to include
    timestamps avoiding the watermark being set earlier than intended.
* maxim_thermocouple
  - Revert a patch adding the max31856 as it's not actually compatible with
    the register set that this driver supports.
* si1133
  - Fix an impossible value check so we don't always error out whether the
    passed in value is good or bad.
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