RDMA: Constify netdev->dev_addr accesses

netdev->dev_addr will become const soon, make sure drivers propagate the
qualifier.

Link: https://lore.kernel.org/r/20211019182604.1441387-4-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Acked-by: Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>