stmhal: Make USB CDC driver use SOF instead of TIM3 for outgoing data.
Previous to this patch the USB CDC driver used TIM3 to trigger the sending of outgoing data over USB serial. This patch changes the behaviour so that the USB SOF interrupt is used to trigger the processing of the sending. This reduces latency and increases bandwidth of outgoing data. Thanks to Martin Fischer, aka @hoihu, for the idea and initial prototype. See PR #1713.