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>