ath9k: dfs_debug fix possible NULL dereference

Fix possible NULL (sc->dfs_detector) pointer dereference.

Detected by Smatch:
drivers/net/wireless/ath/ath9k/dfs_debug.c:67 read_file_dfs()
error: we previously assumed 'sc->dfs_detector' could be null (see line 47)

Signed-off-by: Janusz Dziedzic <janusz.dziedzic@tieto.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>