ar9170: fix build error when !CONFIG_AR9170_LEDS

Fix this build error when CONFIG_AR9170_LEDS is not set
drivers/net/wireless/ath/ar9170/main.c:1296: error: 'struct ar9170' has
no member named 'led_work'

Signed-off-by: Alexander Beregalov <a.beregalov@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>