iwlwifi: mvm: use schedule_delayed_work()

There's no need to refer to system_wq directly, use the provided
wrapper schedule_delayed_work().

Made with the following spatch:
    @@
    expression E,F;
    @@
    -queue_delayed_work(system_wq, E, F);
    +schedule_delayed_work(E, F);

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>