ath9k: store frame information used by aggregation inside the skb tx info

Since the pointers after the rates in the tx info cannot be used anymore
after frames have been queued, this area can be used to store information
that was previously stored in the ath_buf. With these changes, we can delay
the ath_buf assignment in the aggregation code until aggregates are formed.

That will not only make it possible to simplify DMA descriptor setup to
do less rewriting of uncached memory, but will also make it easier to
move aggregation out of the core of the ath9k tx path.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>