tcg/riscv: Remove superfluous breaks

tcg/riscv: Remove superfluous breaks

Remove superfluous breaks, as there is a "return" before them.

Signed-off-by: Liao Pingfang <liao.pingfang@zte.com.cn>
Signed-off-by: Yi Wang <wang.yi59@zte.com.cn>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <1594600421-22942-1-git-send-email-wang.yi59@zte.com.cn>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>