wifi: ath10k: use flexible array in struct wmi_tdls_peer_capabilities

Currently struct wmi_tdls_peer_capabilities defines:
	struct wmi_channel peer_chan_list[1];

Per the guidance in [1] this should be a flexible array, and at one
point Gustavo was trying to fix this [2], but had questions about the
correct behavior when the associated peer_chan_len is 0.

I have been unable to determine if firmware requires that at least one
record be present even if peer_chan_len is 0. But since that is the
current behavior, follow the example from [3] and replace the
one-element array with a union that contains both a flexible array and
a single instance of the array element. This results in a struct that
has the same footprint as the original, so no other driver changes are
required.

No functional changes, compile tested only.

[1] https://docs.kernel.org/process/deprecated.html#zero-length-and-one-element-arrays
[2] https://lore.kernel.org/linux-wireless/626ae2e7-66f8-423b-b17f-e75c1a6d29b3@embeddedor.com/
[3] https://lore.kernel.org/linux-wireless/202308301529.AC90A9EF98@keescook/Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://msgid.link/20231213-wmi_host_mem_chunks_flexarray-v1-5-92922d92fa2c@quicinc.com

drivers/net/wireless/ath/ath10k/wmi.h

@@ -7162,7 +7162,13 @@ struct wmi_tdls_peer_capabilities {
 	__le32 is_peer_responder;
 	__le32 pref_offchan_num;
 	__le32 pref_offchan_bw;
-	struct wmi_channel peer_chan_list[1];
+	union {
+		/* to match legacy implementation allocate room for
+		 * at least one record even if peer_chan_len is 0
+		 */
+		struct wmi_channel peer_chan_min_allocation;
+		DECLARE_FLEX_ARRAY(struct wmi_channel, peer_chan_list);
+	};
 } __packed;
 
 struct wmi_10_4_tdls_peer_update_cmd {