ath11k: fix uninitialized return in ath11k_spectral_process_data()

There is a success path where "ret" isn't initialized where we never
have a ATH11K_SPECTRAL_TAG_SCAN_SEARCH and then ret isn't initialized.

Fixes: 9d11b7b ("ath11k: add support for spectral scan")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200619142922.GA267142@mwanda

drivers/net/wireless/ath/ath11k/spectral.c

@@ -773,6 +773,8 @@ static int ath11k_spectral_process_data(struct ath11k *ar,
 		i += sizeof(*tlv) + tlv_len;
 	}
 
+	ret = 0;
+
 err:
 	kfree(fft_sample);
 unlock: