Coverity reported a missing fall through comment, add it.

Fixes: e5918d7d ("target/rx: TCG translation")
Reported-by: Coverity (CID 1422222 MISSING_BREAK)
Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200403184419.28556-1-philmd@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>