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