iwlwifi: kzalloc txb array

When we allocate queues, we currently don't
use kzalloc() right now. When we then free
those queues again without having used all
entries, we may end up trying to free random
pointers found in the txb array since it was
never initialised. This fixes it simply by
using kzalloc().

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>