iwlwifi: mvm: fix dqa deferred frames marking

When a STA has deferred traffic to TX, an appropriate bit
is turned on in %deferred_tx_frames to indicate deferred
traffic. This marking is never turned off, resulting in
iterating over TIDs with no deferred traffic.

Although this didn't cause any failures/errors/bugs, there
is still no point of iterating over these TIDs when not
needed.

Signed-off-by: Liad Kaufman <liad.kaufman@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>