misc/iwmc3200top: use system_wq instead of dedicated workqueues

With cmwq, there's no reason to use separate workqueues in
iwmc3200top.  Drop them and use system_wq instead.  The used work
items are sync flushed before driver detach.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Tomas Winkler <tomas.winkler@intel.com>