[wlan][mlme][test] Refactor FakeAssocCtx()

No API signature change, but change in how to construct
a return value. The CL induces no change in output either.
Now the IEEE bitfield structs are directly constructed
rather than converted from DDK structs.

Test: Passed unit tests
NET-2007

Change-Id: Ifa56b3bc21fb05574349c57facefce2b716ef8bb
1 file changed