wifi: utils: Get correct channel count

When channel range is configured in scan params, get the
channel count from `chan_idx` instead of taking a difference
of start and end of the channel range. The `difference` method
fails in case of 5GHz band since channels may not be consecutive
numbers.

Signed-off-by: Ravi Dondaputi <ravi.dondaputi@nordicsemi.no>