ath9k: more tx setup cleanups

- remove the BUF_HT flag, and instead check for IEEE80211_TX_CTL_AMPDU
  before calling ath_tx_send_ampdu.
- remove a few unused variables
- calculate frame length before adding the frame padding
- merge the misnamed ath_tx_start_dma function into ath_tx_start
- remove an unused argument for assign_aggr_tid_seqno

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