When BW Expansion feature is enabled using UCI or cfg80211tool
command, the utility function to set/get the BW Expand feature
is introduced inside DFS module.
A new dfs member dfs_use_bw_expand is used to store the status
of BW Expansion feature.
CRs-Fixed: 3229250
Change-Id: If01e080f8e60e883fbeb2d7dfd599b91584bc293