[rdk-b][mt7986][wifi-hal][Refactor get possible channel]

[Description]
Refactor get possible channel.
1. Use iw instead of iwlist
2. Add a check of dfs enable. If dfs is disable, the function should ignore dfs channels.

[Release-log]
N/A

Change-Id: I769f09e2ec6855a1f31505c8077aaef31b9b2315
1 file changed