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>