qcacmn: Update rssi only when its valid

Peer stats struct members initial value 0, so when other functions call
to update peer stats have 0 rssi value which updates mininum
rssi since its the least value. So initialize the rssi with invalid
value initially

Change-Id: Ife033598c7ac47a9a26595f97bff058570d3e91f
CRs-Fixed: 2403735
3 files changed