if_awg: mark the first tx descriptor as ready only after all the other tx descriptors are set up

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.

Submitted by: Guy Yur <guyyur@gmail.com>
Differential Revision: https://reviews.freebsd.org/D13030