[components][usb] fix compile warning in cdc_vcom.c
[components][usb] fix compile warning in cdc_vcom.c

geniusgogo/rt-thread,hezlog/rt-thread,hezlog/rt-thread,hezlog/rt-thread,geniusgogo/rt-thread,RT-Thread/rt-thread,RT-Thread/rt-thread,RT-Thread/rt-thread,geniusgogo/rt-thread,nongxiaoming/rt-thread,geniusgogo/rt-thread,RT-Thread/rt-thread,nongxiaoming/rt-thread,RT-Thread/rt-thread,nongxiaoming/rt-thread,hezlog/rt-thread,nongxiaoming/rt-thread,geniusgogo/rt-thread,nongxiaoming/rt-thread,nongxiaoming/rt-thread,nongxiaoming/rt-thread,RT-Thread/rt-thread,geniusgogo/rt-thread,hezlog/rt-thread,hezlog/rt-thread,hezlog/rt-thread,RT-Thread/rt-thread,geniusgogo/rt-thread