ath10k: fix tx header parsing

Frames are not guaranteed to be 802.11 frames in
ath10k_htt_tx() and the tx completion handler.
In some cases, like TDLS, they can be Ethernet.
Hence checking, e.g. frame_control could yield
bogus results and behavior.

Fortunately this wasn't a real problem so far
because there's no FW/HW combination to encounter
this problem.

However it is good to fix this in advance.

Fixes: 75d85fd9 ("ath10k: introduce basic tdls functionality")
Fixes: eebc67fe ("ath10k: fix pmf for wmi-tlv on qca6174")
Fixes: 7b7da0a0 ("ath10k: drop probe responses when too many are queued")
Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>