Clean up implicit fall through.

Directly use the __attribute__ ((fallthrough)) instead of
a macro.
This was suggested in CL:1772474.

BUG=chromium:997709
TEST=CQ
BRANCH=None

Change-Id: Ic6cd417d7c735395a4b136dbb0879a6f1716da98
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/1897360
Tested-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Commit-Queue: Manoj Gupta <manojgupta@chromium.org>
6 files changed