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.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>