ath10k: move spectral related structures under ath10k debugfs

Spectral related structures are accessed / modified only if ath10k
debugfs is enabled, so it makes more sense to move them under
ATH10K_DEBUGFS

Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>