qcacld-3.0: Fix OOB read in hdd_populate_wifi_signal_info

While handling ll stats information there is a possibility
of OOB read in hdd_populate_wifi_signal_info api as
FW is giving num_chan info as uint_32 and host has
saved into int_32 type variable which may lead to an
extremely large positive number which is getting used as
index of for loop to populate signal info in skp which may
lead to OOB read.

Change-Id: I032eb465db30ee9e3a7ba0c4f017ed781264c47d
CRs-fixed: 2383605