First round of IIO fixes for the 4.17 cycle.

* core
  - fix up some issues with overflow etc around wrong types
    for some fo the kfifo handling functions.  Seems unlikely
    this would be triggered in reality but the fixes are simple
    so let's tidy them up.  Second patch deals with checking
    the userspace value passed for length for potential overflow.
* ad7793
  - Catch up with changes to the ad_sigma_delta core and use
    read_raw / write_raw iwth IIO_CHAN_INFO_SAMP_FEW to handle
    sampling frequency control.
* at91-sama5d2
  - Channel config for differential channels was completely broken.
  - Missing Kconfig dependency for buffer support.
* hid-sensor
  - Fix an issue with powering up after resume due to wrong reference
    counting.
* stm32-dfsdm
  - Fix an issue with second writes of the oversampling settings
    failing.
  - Fix an issue with the sample rate being set to half of requested
    value when particular clock source is used.
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