platform/chrome: cros_ec_sensorhub: Simplify legacy timestamp spreading

On some machines (nami), interrupt latency cause samples to appear
to be from the future and are pegged to the current time.
We would see samples with this pattern:

[t, t + ~5ms, t + ~10ms, t + ~10ms + 100us, t + ~10ms + 200us],
                             (current now)      (current now)
(t is the last timestamp time)

Last 2 samples would be barely spread, causing applications to
complain.

We now spread the entire sequence. This is not great: in the example
the sensor was supposed to send samples every 5ms, it now appears to
send one every 2.5ms, but it is slightly closer to reality:

sampling time in the example above
At sensor level

1             2             3             4            5
+-----5ms-----+-----5ms-----+-----5ms-----+----5ms-----+---> t

Before, at host level
                       1             2             3 4 5
--interrupt delay------+-----5ms-----+-----5ms-----+-+-+---> t

Afer, at host level
                       1       2       3       4       5
--interrupt delay------+-2.5ms-+-2.5ms-+-2.5ms-+-2.5ms-+---> t

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>