With SDIO there's a use after free after a data frame is transfered, call stack
below. This happens because ath10k_htt_tx_hl() directly transmits the skb
provided by mac80211 using ath10k_htc_send(), all other HTT functions use
separate skb created with ath10k_htc_alloc_skb() to transmit the HTC packet.
After the packet is transmitted mac80211 frees the skb in ieee80211_tx_status()
but HTT layer expects that it still owns the skb, and frees it in
ath10k_htt_htc_tx_complete().
To fix this take a reference of skb before sending it to HTC layer to make sure
we still own the skb.
Tested on QCA6174 SDIO with firmware WLAN.RMH.4.4.1-00007-QCARMSWP-1.
ath10k_htt_tx_hl() is only used by SDIO and USB so other busses (PCI, AHB and
SNOC) should be unaffected.