hostap: hostap_ap: Mark 'txt' as __always_unused

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