ath9k: move spectral scan support under a separate config symbol

At the moment, spectral scan support, and with it RELAY, is always enabled
with ATH9K[_HTC]_DEBUGFS. Spectral scan support is currently the only user
of RELAY in ath9k, and it unconditionally reserves a relay channel.

Having debugfs support in ath9k is often useful even on very small embedded
routers, where we'd rather like to avoid the code size and RAM usage of the
relay support.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>