firewire: don't write directly to netdev->dev_addr

Commit 406f42fa ("net-next: When a bond have a massive amount
of VLANs...") introduced a rbtree for faster Ethernet address look
up. To maintain netdev->dev_addr in this tree we need to make all
the writes to it go through appropriate helpers.

Prepare fwnet_hwaddr on the stack and use dev_addr_set() to copy
it to netdev->dev_addr. We no longer need to worry about alignment.
union fwnet_hwaddr does not have any padding and we set all fields
so we don't need to zero it upfront.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>