ath9k: don't trigger spectral scan when not enabled

Doing so enables the FFT generation without prior
configuration, leading to an IRQ storm caused by
invalid (or at least unwanted) PHY errors.

Signed-off-by: Zefir Kurtisi <zefir.kurtisi@neratec.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>