ath10k: don't report unset rssi values to mac80211

ath10k: don't report unset rssi values to mac80211

The SDIO firmware does not provide RSSI value to the host, it's only set to
zero. In that case don't report the value to mac80211. One risk here is that
value zero might be a valid value with other firmware, currently there's no way
to detect that.

Without the fix, the rssi value indicated by iw changes between the actual
value and -95.

Tested with QCA6174 SDIO with firmware WLAN.RMH.4.4.1-00005-QCARMSWP-1.

Co-developed-by: Wen Gong <wgong@codeaurora.org>
Signed-off-by: Alagu Sankar <alagusankar@silex-india.com>
Signed-off-by: Wen Gong <wgong@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>