Refactor the netdev notifier registration into a small helper function.

This is a pre-step towards having mlx5 IB device over an Ethernet port
which doesn't support RoCE. Also, renamed the de-registration helper
and the new helper as netdev notifier and not roce, to make it clear
this is not only used with roce.

This patch doesn't change any functionality.
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Reviewed-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>