qcacld-3.0: set ACS start and end frequency for one channel case

If there is only one channel left after trimming ACS channel list, the
start and end channel frequency in ACS config are not set, which may
result in unexpected behaviors, e.g. the function is_mcc_preferred()
always returns false in such case.
To avoid that, set both frequencies to the one being selected.

Change-Id: If7c4106d2547feb866f1d5139fcb07c921f1d046
CRs-Fixed: 3069590