wifi: iwlwifi: mvm: protect TXQ list manipulation

Some recent upstream debugging uncovered the fact that in
iwlwifi, the TXQ list manipulation is racy.

Introduce a new state bit for when the TXQ is completely
ready and can be used without locking, and if that's not
set yet acquire the lock to check everything correctly.

Reviewed-by: Benjamin Berg <benjamin.berg@intel.com>
Tested-by: Jose Ignacio Tornos Martinez <jtornosm@redhat.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>