IB/ipoib: add get_link_ksettings in ethtool

In order to let the bonding driver report the correct speed
of the underlaying interfaces, when they are IPoIB, the ethtool
function get_link_ksettings() in the IPoIB driver is implemented.

Cc: Joe Jin <joe.jin@oracle.com>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Suggested-by: Håkon Bugge <Haakon.Bugge@oracle.com>
Signed-off-by: Zhu Yanjun <yanjun.zhu@oracle.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>