platform/chrome: cros_ec_sensorhub: Fix EC timestamp overflow

EC is using 32 bit timestamps (us), and before converting it to 64bit
they were not casted, so it would overflow every 4s.
Regular overflow every ~70 minutes was not taken into account either.

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