iwlwifi: mvm: remove per-queue hw refcount

There's no need to have a hw refcount if we just mark the
command queue with a (fake) TID; at that point, the refcount
becomes equivalent to the hweight() of the TID bitmap.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>