iwlwifi: move workqueue to priv

In order to separate the different parts of the
driver better, we are reducing the shared data.
This moves the workqueue to "priv", and removes
it from the transport. To do this, simply use
schedule_work() in the transport.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>