wl12xx: use freezable workqueue for netstack_work

When resuming (after wowlan), we want the rx packets (which is
usually the wake-up packet itself) to be passed to mac80211 only
after the resume notifier was completed, and mac80211 is up and
running (otherwise, the packets will be dropped).

By enqueueing the netstack_work to a freezable workqueue, we can
guarantee the rx processing to occur only after mac80211 was resumed.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Ido Yariv <ido@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>