IB/mlx4: Reorder steps in RoCE GID table initialization

There's no need to reset the gid table twice and we need to do it only
for Ethernet ports. Also, no need to actively scan ndetdevs since it's
being done immediatly after we register netdev notifiers.

Signed-off-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>