In cfg80211_change_iface if we don't get previous mode vdev
references then error is returned. In a case where there is
no vdev existing before this will cause sap failure.
Fix is if previous vdev reference is not available skip vdev
access and continue further operations.
Change-Id: I3209f1e1a86050baf389f20029a97a5bdb50d71d
CRs-Fixed: 2352385