wl12xx: Clean up the dummy packet mechanism

The current implementation allocates a skb each time one is requested by
the firmware. Since dummy packets are handled differently than regular
packets, the skb needs to be marked. Currently, this is done by
setting the pkt_type member to 5. This might not be safe, as we cannot
be sure that there won't be any other packets with this pkt_type value.

Since the packet does not change from one request to another, we can
simply allocate a dummy packet template and always send it. All changes
to the skb done during packet preparation must be reverted, so the same
skb can be reused.

The dummy packets are not transmitted, therefore there's no need to set
the BSSID or our own MAC address.

In addition, the header portion of the packet was zeroed by mistake, so
fix that as well.

Signed-off-by: Ido Yariv <ido@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>