iwlwifi: mvm: don't transmit on unallocated queue

We can currently end up transmitting on an unallocated queue, if
the allocation fails. Stop doing that, by simply not transmitting.
We don't have any better strategy here, unfortunately, but the
previous commits make that much less likely.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20200425130140.dcf1801f25ef.I6d71e13ea042765800f2ee41401b8eb282527c34@changeid