qcacld-3.0: Add a sanity check to prevent integer overflow

Currently in the function hdd_send_roam_scan_channel_freq_list_to_sme,
the num_chan variable is declared as uint8_t and is incremented
for each nested attribute PARAM_SCAN_FREQ_LIST.

If the number of attributes sent by userspace is more than max value
of uint8_t, then an integer overflow occurs.

To avoid this issue, add a sanity check to see if num_chan has reached
SIR_MAX_SUPPORTED_CHANNEL_LIST before incrementing variable.

Change-Id: I601a73a118eb65ebb8575f6ed5ed1f29d915f59e
CRs-Fixed: 3568577