STPK-1429 Slight improvement to timestamp accuracy

STPK-1429 Slight improvement to timestamp accuracy

Use date immediately before the iio device read operation instead
of sensor report propagation time as the event timestamp. That's
only an intermediate step towards higher precision timestamps
though, as some drivers expose a timestamp channel we can read at
the same time as associated sample data.

Issue: STPK-1429

Change-Id: Iad5e1dc35fcbd422d3d1390cab202e3c36de8504
Signed-off-by: Patrick Porlan <patrick.porlan@intel.com>