set_primary_vdev usecase in STA+STA concurrency is to use that
interface(e.g. wlan0) over the other interface(i.e. wlan1) for
data transfer.
Non-primary vdev use case is to check the quality of that link
and decide if data can be switched to it and make it primary.
It may get disabled if it's not going to become primary STA after
measuring the link quality for sometime or with few APs.
Enable roaming on non-primary vdev also to find better AP.
Change-Id: I3205f1ee1b7da4aa813955308730949ccc70941f
CRs-Fixed: 3219633
7c1083a4ba