mwl8k: fix addr4 zeroing and payload overwrite on DMA header creation

When inserting a DMA header into a packet for transmission,
mwl8k_add_dma_header() would blindly zero the addr4 field, which
is not a good idea if the packet being transmitted is actually a
4-address packet.

Also, if the transmitted packet was a 4-address with QoS packet,
the memmove() to do the needed header reshuffling would inadvertently
overwrite the first two bytes of the packet payload with the QoS field.

This fixes both of these issues.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>