staging: rtl8723au: Use a workqueue for command handling

Rewrite the old thread based code to use a workqueue instead. This
removes a pile of complexity, locks and queues and eliminates problem
that cmd handling could have been killed from userspace.

This was suggested by Tejun Heo - thanks!

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>