iwlwifi: mvm: allow private per-STA TFD queues

TDLS stations will have private queues, so consider them as well when
allocating a new one. Consolidate the HW-queue iterating code into
a single exported function, to be used by the TDLS code in the future.

Signed-off-by: Arik Nemtsov <arikx.nemtsov@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>