ath9k: Only remove spectral scan relay file when it was created

The relay file depends on relayfs. Trying to close this file without having
ATH9K_DEBUGFS (and therefore RELAY) activated causes build failures.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>