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>