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

dp/inc/cdp_txrx_stats_struct.h

@@ -118,6 +118,8 @@
 #define CDP_FC_IS_RETRY_SET(_fc) \
 	((_fc) & qdf_cpu_to_le16(CDP_FCTL_RETRY))
 
+#define INVALID_RSSI 255
+
 /* Different Packet Types */
 enum cdp_packet_type {
 	DOT11_A = 0,