Fix accounting of the number of frames queued on an iwx(4) Tx ring.

Fix accounting of the number of frames queued on an iwx(4) Tx ring.

Stop decrementing ring->queued inside the if-statement which guards
maintenance of the OACTIVE flag. This is wrong and resulted in a negative
counter value (visible in firmware error traces). The counter is already
decremented in the loop above where frames are taken off the ring.