qcacld-3.0: Always append preferred roam scan channels

For ADDROAMSCANCHANNELS command, always append user given channels
into the existing preferred channels list in driver. If the preferred
channel list exceeds SIR_ROAM_MAX_CHANNELS i.e. 80 then overwrite
oldest entries with new channels.

Change-Id: Ifa12b1e4d75dd927bdbceea9a076ed1bcbd91f4f
CRs-Fixed: 2618318