ath: fix build break with ATH_DBG_WARN_ON_ONCE

Description by Hauke:

"If CONFIG_ATH_DEBUG=y is set ATH_DBG_WARN_ON_ONCE uses WARN_ON_ONCE and
returns something, but if CONFIG_ATH_DEBUG is not set it does not return
anything. Now ATH_DBG_WARN_ON_ONCE is used in the boolean expression in
an if case and is not returning anything and causes a compile error.

  CC [M]  /drivers/net/wireless/ath/ath9k/main.o
/drivers/net/wireless/ath/ath9k/main.c: In function ‘ath_isr’:
/drivers/net/wireless/ath/ath9k/main.c:769: error: expected expression
before ‘do’
make[5]: *** [/drivers/net/wireless/ath/ath9k/main.o] Error 1
make[4]: *** [/drivers/net/wireless/ath/ath9k] Error 2"

Reported-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath/ath.h

@@ -272,7 +272,10 @@ ath_dbg(struct ath_common *common, enum ATH_DEBUG dbg_mask,
 	return 0;
 }
 #define ATH_DBG_WARN(foo, arg...) do {} while (0)
-#define ATH_DBG_WARN_ON_ONCE(foo) do {} while (0)
+#define ATH_DBG_WARN_ON_ONCE(foo) ({				\
+	int __ret_warn_once = !!(foo);				\
+	unlikely(__ret_warn_once);				\
+})
 
 #endif /* CONFIG_ATH_DEBUG */