net: qmi_wwan: use dev_addr_mod()

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 got through appropriate helpers.

Acked-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>