wifi: iwlwifi: mvm: check link during TX

Again, during some (botched) FW restart scenarios we can end
up with a NULL link in the driver but mac80211 thinking all
is still going OK. If we try to TX at the same time, we can
crash there. Fix that by checking for a NULL link during TX.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230614123446.cee48479deec.I4eef58f7b67afafb7b3294adbeb6e0067b68419d@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>