libevdev: enforce state resync if SYN_DROPPED happens

BUG=chrome-os-partner:20471
TEST=none; don't know how to trigger it yet

Change-Id: Icfdafb64ceb7e7ac27ad80b16570a9f3c4d503e2
Reviewed-on: https://gerrit.chromium.org/gerrit/60209
Reviewed-by: Andrew de los Reyes <adlr@chromium.org>
Commit-Queue: Chung-yih Wang <cywang@chromium.org>
Tested-by: Chung-yih Wang <cywang@chromium.org>