wifi: ath10k: replace deprecated strncpy with memcpy

strncpy() is deprecated [1] and we should prefer less ambiguous
interfaces.

In this case, arvif->u.ap.ssid has its length maintained by
arvif->u.ap.ssid_len which indicates it may not need to be
NUL-terminated. Make this explicit with __nonstring and use a plain old
memcpy.

This is also consistent with future copies into arvif->u.ap.ssid:

	if (changed & BSS_CHANGED_SSID &&
	    vif->type == NL80211_IFTYPE_AP) {
		arvif->u.ap.ssid_len = vif->cfg.ssid_len;
		if (vif->cfg.ssid_len)
			memcpy(arvif->u.ap.ssid, vif->cfg.ssid,
			       vif->cfg.ssid_len);
		arvif->u.ap.hidden_ssid = info->hidden_ssid;
	}

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
Acked-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20231024-strncpy-drivers-net-wireless-ath-ath10k-mac-c-v2-1-4c1f4cd4b4df@google.com

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

@@ -607,7 +607,7 @@ struct ath10k_vif {
 			u8 tim_bitmap[64];
 			u8 tim_len;
 			u32 ssid_len;
-			u8 ssid[IEEE80211_MAX_SSID_LEN];
+			u8 ssid[IEEE80211_MAX_SSID_LEN] __nonstring;
 			bool hidden_ssid;
 			/* P2P_IE with NoA attribute for P2P_GO case */
 			u32 noa_len;

drivers/net/wireless/ath/ath10k/mac.c

@@ -6121,9 +6121,8 @@ static void ath10k_bss_info_changed(struct ieee80211_hw *hw,
 
 		if (ieee80211_vif_is_mesh(vif)) {
 			/* mesh doesn't use SSID but firmware needs it */
-			strncpy(arvif->u.ap.ssid, "mesh",
-				sizeof(arvif->u.ap.ssid));
 			arvif->u.ap.ssid_len = 4;
+			memcpy(arvif->u.ap.ssid, "mesh", arvif->u.ap.ssid_len);
 		}
 	}