wifi: ath9k: cleanup struct ath_tx_control and ath_tx_prepare()

wifi: ath9k: cleanup struct ath_tx_control and ath_tx_prepare()

After switching to mac80211 software queues, pointer to 'struct ath_node'
in 'struct ath_tx_control' is still assigned but not actually used. So drop
it and cleanup related things in 'ath_tx_prepare()'. Compile tested only.

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Link: https://patch.msgid.link/20250115171750.259917-1-dmantipov@yandex.ru
Signed-off-by: Jeff Johnson <jeff.johnson@oss.qualcomm.com>