hostap: hostap_ap: Mark 'txt' as __always_unused

Keeping this around to act as documentation, since its use is
currently #if'ed out at the end of the function.

Fixes the following W=1 kernel build warning(s):

 drivers/net/wireless/intersil/hostap/hostap_ap.c: In function ‘handle_assoc’:
 drivers/net/wireless/intersil/hostap/hostap_ap.c:1507:8: warning: variable ‘txt’ set but not used [-Wunused-but-set-variable]

Cc: Jouni Malinen <j@w1.fi>
Cc: Kalle Valo <kvalo@codeaurora.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Nathan Chancellor <natechancellor@gmail.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: linux-wireless@vger.kernel.org
Cc: netdev@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200819072402.3085022-12-lee.jones@linaro.org

drivers/net/wireless/intersil/hostap/hostap_ap.c

@@ -1504,7 +1504,7 @@ static void handle_assoc(local_info_t *local, struct sk_buff *skb,
 	u16 resp = WLAN_STATUS_SUCCESS;
 	struct sta_info *sta = NULL;
 	int send_deauth = 0;
-	char *txt = "";
+	char __always_unused *txt = "";
 	u8 prev_ap[ETH_ALEN];
 
 	left = len = skb->len - IEEE80211_MGMT_HDR_LEN;