Add comment about forcing 12 Mbit/sec USB on Teensy 4.x

teensy4/usb.c

 	USB1_USBCMD = USB_USBCMD_RS;
 	//transfer_log_head = 0;
 	//transfer_log_count = 0;
+	//USB1_PORTSC1 |= USB_PORTSC1_PFSC; // force 12 Mbit/sec
 }