ath5k: remove ts_retry from ath5k_tx_status

Reusing the configured retry counts from the skb cb is more efficient than
reloading the data from uncached memory.
Replace ts_longretry (unused) with ts_final_retry which contains the retry
count for the final rate only

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