Input: reset device timestamp on sync

We need to reset input device's timestamp on input_sync(), otherwise
drivers not using input_set_timestamp() will end up with a stale
timestamp after their clients consume first input event.

Fixes: 3b51c44b ("Input: allow drivers specify timestamp for input events")
Reported-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>