IB/mlx4: Avoid null pointer dereference in mlx4_ib_scan_netdevs()

When Ethernet netdev is not present for a port (e.g. when the link
layer type of the port is InfiniBand) it's possible to dereference a
null pointer when we do netdevice scanning.

To fix that, we move a section of code that needs to run only when
netdev is present to a proper if () statement.

Fixes: ad4885d2 ("IB/mlx4: Build the port IBoE GID table properly under bonding")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>