mwifiex: unregister wiphy before freeing resources

It's possible for some control interfaces (e.g., scans, set freq) to be
active after we've stopped our main work queue and the netif TX queues.
These don't get completely shut out until we've unregistered the wdevs
and wiphy.

So let's only free command buffers and poison our lists after
wiphy_unregister().

This resolves various use-after-free issues seen when resetting the
device.

Cc: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>