wl12xx: don't indicate up PS-filtered dummy packets

Dummy packets are currently only sent on the system_hlid link. The
system_hlid link should never be filtered for PS (as it is not
a STA link). Even so, for correctness, don't indicate dummy packets up.
The skb does not belong to mac80211 and as such does not contain a
correct skb->cb.

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