wl12xx: don't queue a new dummy packet if one is already pending
The firmware only asks for one dummy packet at a time, but sometimes we
are unable to provide it before a FW timer expires. When this happens,
the FW will re-request the dummy packet. If a packet is still queued in
the driver queues, do nothing in this case.
This prevents spurious dummy packets from clogging up the VO AC.