Enable atheros fft support

Add required configuration:

CONFIG_ATH9K_COMMON_SPECTRAL CONFIG_ATH9K_DEBUGFS

References:

• https://wireless.docs.kernel.org/en/latest/en/users/drivers/ath9k/spectral_scan.html
• https://wireless.docs.kernel.org/en/latest/en/users/drivers/ath10k/spectral.html