wlcore: do not report noise level in get survey op

The get survey op expects the low level driver to report
the noise level for a a given channel.

The noise calculated in wlcore is (rssi-snr/2), but since
the snr reported by the FW is a derivative from the rssi
this calculation is useless, and should not be reported
to the user space.

Reporting incorrect noise, results in the wpa_supplicant
miscalculating the roaming candidate priority, thus causing
a situation where an AP with a lower rssi level would be
chosen over a better AP.

Signed-off-by: Yoni Divinsky <yoni.divinsky@ti.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>