ath10k: move spectral scan support under a separate config symbol

At the moment, spectral scan support, and with it RELAY, is always enabled
with ATH10K_DEBUGFS. Spectral scan support is currently the only user of
RELAY in ath10k, and it unconditionally reserves a relay channel.

Having debugfs support in ath10k 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. While ath10k-based devices usually have more
resources than ath9k-based ones, it makes sense to keep the configuration
symmetric to ath9k, so the same base kernel without RELAY can be used for
both ath9k and ath10k hardware.

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