ath10k: htt_tx: do not interpret Eth frames as WiFi

The xmit path for the Ethernet encapsulated frames become more or less
usable since d740d8fd2439 ("ath10k: unify tx mode and dispatch"). This
change reorganize the xmit path in a manageable way to properly support
various tx modes, but misses that the Ethernet encapsulated frame is a
special case. We do not have an IEEE 802.11 header at the begining of
them. But the HTT Tx handler still interprets first bytes of each frame
as an IEEE 802.11 Frame Control field.

Than this code was copied by e62ee5c381c5 ("ath10k: Add support for
htt_data_tx_desc_64 descriptor") and a2097d6444c3 ("ath10k: htt: High
latency TX support") to another handlers. In fact the issue in the high
latency (HL) handler was introduced by 83ac260151e7 ("ath10k: add mic
bytes for pmf management packet").

Ethernet encapsulated frame tx mode stay unused until 75d85fd9993c
("ath10k: introduce basic tdls functionality") started using it for TDLS
frames to avoid key selection issue in some firmwares.

Trying to interpret the begining of an Ethernet encapsulated frame as an
IEEE 802.11 header was not hurt us noticeably since we need to meet two
conditions: (1) xmit should be performed towards a TDLS peer, and (2)
the TDLS peer should have a specific OUI part of its MAC address. Looks
like that the rareness in TDLS communications of OUIs that can be
interpreted as an 802.11 management frame saves users from facing this
issue earlier.

Improve Ethernet tx mode support in the HTT Tx handler by avoiding
interpreting its first bytes as an IEEE 802.11 header. While at it, make
the ieee80211_hdr variable local to the code block that is guarded by
!is_eth check. In this way, we clarify in which cases a frame can be
interpreted as IEEE 802.11, and saves us from similar issues in the
future.

Credits: this change as part of xmit encapsulation offloading support
was originally made by QCA and then submitted for inclusion by John
Crispin [1]. But the whole work was not accepted due to the lack of a
part for 64-bits descriptors [2]. Zhijun You then pointed this out to me
in a reply to my initial RFC patch series. And I made this slightly
reworked version that covered all the HTT Tx handler variants.

1. https://lore.kernel.org/all/20191216092207.31032-1-john@phrozen.org/
2. https://patchwork.kernel.org/project/linux-wireless/patch/20191216092207.31032-1-john@phrozen.org/

Reported-by: Zhijun You <hujy652@gmail.com>
Signed-off-by: Vasanthakumar Thiagarajan <vthiagar@qti.qualcomm.com>
Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220516032519.29831-3-ryazanov.s.a@gmail.com

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