wifi: ath12k: Avoid -Wflex-array-member-not-at-end warnings

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Move the conflicting declaration to the end of the structure. Notice
that `struct ieee80211_chanctx_conf` is a flexible structure --a
structure that contains a flexible-array member.

Also, remove an unused structure.

Fix the following warnings:
drivers/net/wireless/ath/ath12k/core.h:290:39: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
drivers/net/wireless/ath/ath12k/dp.h:1499:24: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://patch.msgid.link/ZrZEuxJihMzAaTVh@cute