wl12xx: handle injected packets

Injected packets are sent with no vif, causing the wl12xx
to NULL-dereference in multiple places.

Furthermore, injected packets are currently not sent at all,
as system_hlid doesn't belong to any specific role, so
wl1271_skb_dequeue() never return its packets.

Handle both these problems.

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