qcacmn: Add API to get SNR value from scan entry

Add snr and avg_snr variables to scan entry and populate these variables
while generating the scan entry.
Add API to read avg_snr value from the scan entry.

Change-Id: Ie19d5ad7c8debb35e9d278ec648bd6217b2f4099
CRs-Fixed: 2532369
4 files changed