wireless/ipw2x00: use system_wq instead of dedicated workqueues

With cmwq, there's no reason to use separate workqueues in ipw2x00
drivers.  Drop them and use system_wq instead.  All used work items
are sync canceled on driver detach.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: "John W. Linville" <linville@tuxdriver.com>
Cc: linux-wireless@vger.kernel.org