qualcommax: ipq807x: ipq6018: ath11k: fix monitor rx length

qualcommax: ipq807x: ipq6018: ath11k: fix monitor rx length

OpenWrt issue:
https://github.com/openwrt/openwrt/issues/16183

Problem summary:
On qualcommax (ipq60xx/ipq807x) with ath11k, monitor-mode captures contain
frames that are consistently longer than expected by 8 bytes.

The symptom is visible in pcap/radiotap captures, and Wireshark parsing
becomes correct after manually cutting these 8 bytes from captured frames.

This patch:
- Remove merge-stage FCS/tail manipulations in ath11k_dp_rx_mon_merg_msdus().
- add length fix in ath11k_dp_rx_mon_deliver(), trim 8 bytes right
  before radiotap update and delivery to mac80211.

This targets monitor capture length correctness only and keeps the fix scoped
to the monitor RX delivery path.

Tested-on: ipq8072 yuncore,ax880; ipq6018 yuncore,ax840; yuncore,fap650
Signed-off-by: Ruslan Isaev <legale.legale@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/22244
Signed-off-by: Robert Marko <robimarko@gmail.com>