Be more careful with the amount of data we read from the iio dev fd

Be more careful with the amount of data we read from the iio dev fd

The iio drivers we have operate with a buffer length set at 2, and
can be set at larger sizes, however the current event integration
code path only handles a iio device report per loop.

For now, simply make sure we only read a single "scan", a.k.a. iio
device report, per loop. This could avoid dropping events if the
driver is producing them faster than we can consume them.

Change-Id: Ibfad7abbd445d6aae0345e2c828ca72c555c42ea
Signed-off-by: Patrick Porlan <patrick.porlan@intel.com>