ath9k: defer ath_tx_setup_buffer setup to the first tx attempt during aggr

With sequence number and buffer allocation deferred to when they're needed
for the first time, it becomes much easier to start dropping packets from
the tid queue if necessary, e.g. when latency suddenly increases. This can
lead to some future improvements in buffer management for better latency.

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