Fixes stuttering output.
The use of the semaphore was causing blocking
on non blocking callers. This ensured that
the TX DAM would be restated, but when it
was switched to trywait in 8362e314, it left
data in the xmit queue unsent.
This solution removes the semaphore and restart
the DMA on completion if there is more data in
the xmit queue to be sent.
9175f3a9b6