hostap: Mark 'freq_list' as __maybe_unused

'freq_list' is used in some source files which include hostap.h, but
not all.  The compiler complains about the times it's not used.  Mark
it as __maybe_used to tell the compiler that this is not only okay,
it's expected.

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

 In file included from drivers/net/wireless/intersil/hostap/hostap_80211_tx.c:9:
 drivers/net/wireless/intersil/hostap/hostap.h:11:19: warning: ‘freq_list’ defined but not used [-Wunused-const-variable=]
 In file included from drivers/net/wireless/intersil/hostap/hostap_main.c:31:
 drivers/net/wireless/intersil/hostap/hostap.h:11:19: warning: ‘freq_list’ defined but not used [-Wunused-const-variable=]
 In file included from drivers/net/wireless/intersil/hostap/hostap_proc.c:10:
 drivers/net/wireless/intersil/hostap/hostap.h:11:19: warning: ‘freq_list’ defined but not used [-Wunused-const-variable=]
 In file included from drivers/net/wireless/intersil/hostap/hostap_hw.c:50,
 from drivers/net/wireless/intersil/hostap/hostap_cs.c:196:
 At top level:
 drivers/net/wireless/intersil/hostap/hostap.h:11:19: warning: ‘freq_list’ defined but not used [-Wunused-const-variable=]
 In file included from drivers/net/wireless/intersil/hostap/hostap_hw.c:50,
 from drivers/net/wireless/intersil/hostap/hostap_pci.c:221:
 At top level:
 drivers/net/wireless/intersil/hostap/hostap.h:11:19: warning: ‘freq_list’ defined but not used [-Wunused-const-variable=]
 In file included from drivers/net/wireless/intersil/hostap/hostap_hw.c:50,
 from drivers/net/wireless/intersil/hostap/hostap_plx.c:264:
 At top level:
 drivers/net/wireless/intersil/hostap/hostap.h:11:19: warning: ‘freq_list’ defined but not used [-Wunused-const-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: 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/20200819072402.3085022-2-lee.jones@linaro.org