ath9k: move the sequence number from ath_frame_info to ath_buf

It is only necessary for BAW tracking and moving it to the ath_buf
makes it easier to add further improvements, such as deferring
seqno allocation in the aggregation path.

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