iwlwifi: mvm: fix DQA AP mode station assumption

There was recently a Full AP mode feature added where hostap adds
the station at auth stage, and not assoc.
However, when running with legacy hostapd, we get the auth response
before station was added, and tx_skb_non_sta fails to allocate
a queue, resulting in a complete failure of association.
Take care of this situation as well.
Add a warning when no valid queue is returned at all and make mvm
drop the packet instead of passing it on.
Refactor the function a bit while at it.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>