if_awg: store mbuf and dma mapping in the last segment of a tx frame instead of the first

[guyyur_gmail.com]

Summary

According to the datasheet, TX_DESC_CTL is cleared when whole frame is transmitted or all
data in the current descriptor's buffer are transmitted.
When the mbuf and mapping are stored in the first segment and in a scenario where a tx
completion interrupt arrives for a frame and only the start of the next frame was transmitted,
at the time of interrupt processing the mbuf and mapping will be freed when processing the
first segment of the next frame but the other untrasmitted segments still need to use them.