devlink: Move netdev_to_devlink helpers to devlink.c

Both netdev_to_devlink and netdev_to_devlink_port are used in devlink.c
only, so move them in order to reduce their scope.

Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>