ath10k: make ath10k report discarded packets to mac80211

Whenever ath10k firmware discards a packet (HTT_TX_COMPL_STATE_DISCARD
flag), the skb is freed and mac80211 does not get feedback through
ieee80211_tx_status().

Instead, make sure that the IEEE80211_TX_STAT_ACK flag is disabled and
let the packet go through, like ath9k does.

Signed-off-by: Ignacio Nunez Hernanz <nacho.nunez@aoifes.com>
[kvalo@codeaurora.org: rebase patch manually]
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>