staging: vt6655: vnt_tx_packet fix dma_idx selection.

There is still a problem that dma_idx is causing packets to
go onto the wrong tx path.

Protect dma_idx fully with the present first lock and
use pTDInfo->byFlags TD_FLAGS_NETIF_SKB to set MACvTransmit.

Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>