Staging driver bugfixes for 3.16-rc4

Nothing major here, just 4 small bugfixes that resolve some issues
reported for the IIO (staging and non-staging) and the tidspbridge
driver.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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Merge tag 'iio-fixes-for-3.16b' of git://git.kernel.org/pub/scm/linux/kernel/git/jic23/iio into staging-linus

Jonathan writes:

Second set of IIO fixes for the 3.16 cycle.

* A fix for a bug in setting threshold levels within the ad799x driver
  which prevents correct setting of the thresholds.
* In ad7291 fix an case where a ERR_PTR value was returned directly
  instead of having PTR_ERR applied.  Hence it would report success
  instead of failure.
* of_iio_channel_get_by_name returned a non null pointer if it fails
  and the callee was expecting NULL to indicate failure.  Fixed by
  returning NULL in the error cases.