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>