Fix race condition in USB CDC transmit

If the Start of Frame interrupt triggers just after the call
to USB_SendSpace in USB_Send then we can get data loss.
When the first bank is full and the second partially full,
the SOF handler will release the second bank via USB_Flush.
Data is then lost due to overflow as USB_Send continues writing data
to the now-closed bank.

Fix this by re-checking the FIFO status inside LockEP, immediately before
doing the data write.

Signed-off-by: Paul Brook <paul@nowt.org>

cores/arduino/USBCore.cpp

@@ -290,9 +290,12 @@ int USB_Send(u8 ep, const void* d, int len)
 
 		if (n > len)
 			n = len;
-		len -= n;
 		{
 			LockEP lock(ep);
+			// Frame may have been released by the SOF interrupt handler
+			if (!ReadWriteAllowed())
+				continue;
+			len -= n;
 			if (ep & TRANSFER_ZERO)
 			{
 				while (n--)