wlcore: ignore dummy packet events in PLT mode

Sometimes the firmware sends a dummy packet event while we are in PLT
mode.  This doesn't make sense, it's a firmware bug.  Fix this by
ignoring dummy packet events when we're PLT mode.

Reported-by: Yegor Yefremov <yegorslists@googlemail.com>
Reported-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <luca@coelho.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>