cdc_vcom: not to start sending data when the data is already sending
cdc_vcom: not to start sending data when the data is already sending

We start the sending transaction in SOF handler. But if the data is
already sending, start an other transaction will cause data lose.
Implement a state machine is cdc_vcom and avoid that.

hezlog/rt-thread,weiyuliang/rt-thread,RT-Thread/rt-thread,igou/rt-thread,igou/rt-thread,hezlog/rt-thread,wolfgangz2013/rt-thread,gbcwbz/rt-thread,nongxiaoming/rt-thread,hezlog/rt-thread,FlyLu/rt-thread,geniusgogo/rt-thread,FlyLu/rt-thread,FlyLu/rt-thread,zhaojuntao/rt-thread,AubrCool/rt-thread,armink/rt-thread,nongxiaoming/rt-thread,gbcwbz/rt-thread,weety/rt-thread,armink/rt-thread,weiyuliang/rt-thread,RT-Thread/rt-thread,FlyLu/rt-thread,weiyuliang/rt-thread,nongxiaoming/rt-thread,igou/rt-thread,armink/rt-thread,weiyuliang/rt-thread,ArdaFu/rt-thread,hezlog/rt-thread,AubrCool/rt-thread,FlyLu/rt-thread,geniusgogo/rt-thread,weiyuliang/rt-thread,weiyuliang/rt-thread,weety/rt-thread,AubrCool/rt-thread,igou/rt-thread,weety/rt-thread,nongxiaoming/rt-thread,yongli3/rt-thread,wolfgangz2013/rt-thread,zhaojuntao/rt-thread,AubrCool/rt-thread,RT-Thread/rt-thread,hezlog/rt-thread,armink/rt-thread,hezlog/rt-thread,yongli3/rt-thread,ArdaFu/rt-thread,wolfgangz2013/rt-thread,armink/rt-thread,ArdaFu/rt-thread,ArdaFu/rt-thread,weety/rt-thread,yongli3/rt-thread,weety/rt-thread,geniusgogo/rt-thread,zhaojuntao/rt-thread,AubrCool/rt-thread,zhaojuntao/rt-thread,nongxiaoming/rt-thread,armink/rt-thread,ArdaFu/rt-thread,gbcwbz/rt-thread,weiyuliang/rt-thread,geniusgogo/rt-thread,zhaojuntao/rt-thread,igou/rt-thread,yongli3/rt-thread,wolfgangz2013/rt-thread,geniusgogo/rt-thread,FlyLu/rt-thread,igou/rt-thread,weety/rt-thread,gbcwbz/rt-thread,hezlog/rt-thread,geniusgogo/rt-thread,yongli3/rt-thread,zhaojuntao/rt-thread,gbcwbz/rt-thread,FlyLu/rt-thread,igou/rt-thread,nongxiaoming/rt-thread,yongli3/rt-thread,RT-Thread/rt-thread,wolfgangz2013/rt-thread,weety/rt-thread,wolfgangz2013/rt-thread,gbcwbz/rt-thread,ArdaFu/rt-thread,wolfgangz2013/rt-thread,zhaojuntao/rt-thread,AubrCool/rt-thread,ArdaFu/rt-thread,geniusgogo/rt-thread,yongli3/rt-thread,gbcwbz/rt-thread,armink/rt-thread,AubrCool/rt-thread,RT-Thread/rt-thread,RT-Thread/rt-thread,nongxiaoming/rt-thread,RT-Thread/rt-thread