[wlan][mlme] Omits 0 rssi frames from average.

This fixes the huge volume of bad readings seen due to unimplemented
wlan_rx_info for data frames. Subsequent work should resolve these instances
of missing info.

WLAN-1018

Test=ran existing unit tests and verified histogram fixed in `wlan stats`

Change-Id: I3395b65c4060664f20a3bdff0d08b03fbc8458cd