qcacld-3.0: Avoid race when roaming to invalid channel

Currently, when channel list is changed, it sends the
channel list to fw first and then disconnect the sta.
This causes race condition to roam to invalid channel.

To resolve this, disconnect the sta first and then update
channel list.

Change-Id: I49eddf5a2b64739993e420eac99caaf6e8180a72
CRs-fixed: 2806706