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: 9d11b7bf

 ("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