ath11k: Add support spectral scan for IPQ6018

IPQ6018 supported with 4 bytes FFT BIN size. so supported 4 bytes
parsing logic in FFT report process. since spectral_fft_sz is
configured as zero in hw_params, spectral is not supported in
QCA6390 platform.

Tested-on: IPQ6018 WLAN.HK.2.1.0.1-01228-QCAHKSWPL_SILICONZ-1

Signed-off-by: Karthikeyan Periyasamy <periyasa@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1600236776-4042-1-git-send-email-periyasa@codeaurora.org