RDMA/ipoib: implement ethtool .get_link() callback

Add support for reporting link state for ipoib net devices.

$ ip l set dev mlx4_ib0 up
$ ethtool mlx4_ib0 | grep Link
	Link detected: yes
$ ip l set dev mlx4_ib0 down
$ ethtool mlx4_ib0 | grep Link
	Link detected: no

Signed-off-by: Kamal Heib <kamalheib1@gmail.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/ulp/ipoib/ipoib_ethtool.c

@@ -222,6 +222,7 @@ static const struct ethtool_ops ipoib_ethtool_ops = {
 	.get_strings		= ipoib_get_strings,
 	.get_ethtool_stats	= ipoib_get_ethtool_stats,
 	.get_sset_count		= ipoib_get_sset_count,
+	.get_link		= ethtool_op_get_link,
 };
 
 void ipoib_set_ethtool_ops(struct net_device *dev)