seems to work like a tag that indicates 'not list end' of queued
frames. Without having a VGE_TXDESC_Q bit indicates 'list end'. So
the last frame of multiple queued frames has no VGE_TXDESC_Q bit.
The hardware has peculiar behavior for VGE_TXDESC_Q bit handling.
If the VGE_TXDESC_Q bit of descriptor was set the controller would
fetch next descriptor. However if next descriptor's OWN bit was
cleared but VGE_TXDESC_Q was set, it could confuse controller.
Clearing VGE_TXDESC_Q bit for transmitted frames ensure correct
behavior.
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