In a multi segment frame, if the first tx descriptor is marked with TX_DESC_CTL
but not all tx descriptors for the other segments in the frame are set up,
the TX DMA may transmit an incomplete frame.
To prevent this, set TX_DESC_CTL for the first tx descriptor only when done
with all the other segments.

Also, don't bother cleaning transmitted tx descriptors since TX_DESC_CTL
is cleared for them by the hardware and they will be reprogrammed before
TX_DESC_CTL is reenabled for them.