qcacld-3.0: Avoid null pointer access of vdev

In function hdd_softap_set_channel_change, vdev is passed to
function wlan_vdev_mlme_get_opmode without taking reference.
This can lead to NULL pointer access.

Change-Id: Ibbc58a1e7a4be0e0e34982b99541a63cd77e0480
CRs-Fixed: 2518560