iwlwifi: mvm: ignore STOP_AGG when restarting

Since the device is being restarted, all the Rx / Tx Block
Ack sessions are been wiped out by the driver. So ignore
the requests from mac80211 that stops Tx agg while
reconfiguring the device.

Note that stopping a non-existing Rx BA session is harmless,
so just honor mac80211's request.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>