First round of IIO fixes for the 4.14 cycle

Note this includes fixes from recent merge window.  As such the tree
is based on top of a prior staging/staging-next tree.

* iio core
  - return and error for a failed read_reg debugfs call rather than
    eating the error.
* ad7192
  - Use the dedicated reset function in the ad_sigma_delta library
    instead of an spi transfer with the data on the stack which
    could cause problems with DMA.
* ad7793
  - Implement a dedicate reset function in the ad_sigma_delta library
    and use it to correctly reset this part.
* bme280
  - ctrl_reg write must occur after any register writes
  for updates to take effect.
* mcp320x
  - negative voltage readout was broken.
  - Fix an oops on module unload due to spi_set_drvdata not being called
    in probe.
* st_magn
  - Fix the data ready line configuration for the lis3mdl. It is not
    configurable so the st_magn core was assuming it didn't exist
    and so wasn't consuming interrupts resulting in an unhandled
    interrupt.
* stm32-adc
  - off by one error on max channels checking.
* stm32-timer
  - preset should not be buffered - reorganising register writes avoids
  this.
  - fix a corner case in which write preset goes wrong when a timer is
  used first as a trigger then as a counter with preset. Odd case but
  you never know.
* ti-ads1015
  - Fix setting of comparator polarity by fixing bitfield definition.
* twl4030
  - Error path handling fix to cleanup in event of regulator
    registration failure.
  - Disable the vusb3v1 regulator correctly in error handling
  - Don't paper over a regulator enable failure.
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