ath9k: fix sparse non static symbol warning

Fixes the following sparse warning:

drivers/net/wireless/ath/ath9k/spectral.c:500:24: warning:
 symbol 'rfs_spec_scan_cb' was not declared. Should it be static?
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath/ath9k/spectral.c

@@ -497,7 +497,7 @@ static int remove_buf_file_handler(struct dentry *dentry)
 	return 0;
 }
 
-struct rchan_callbacks rfs_spec_scan_cb = {
+static struct rchan_callbacks rfs_spec_scan_cb = {
 	.create_buf_file = create_buf_file_handler,
 	.remove_buf_file = remove_buf_file_handler,
 };