iwlwifi: pcie: wake the queue if stopped when being unmapped

When the queue is unmapped while it was so loaded that
mac80211's was stopped, we need to wake the queue after
having freed all the packets in the queue.
Not doing so can result in weird stuff like:

* run lots of traffic (mac80211's queue gets stopped)
* RFKILL
* de-assert RFKILL
* no traffic

Cc: stable@vger.kernel.org
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>