IB/mlx4: Use correct SL on AH query under RoCE

The mlx4 IB driver implementation for ib_query_ah used a wrong offset
(28 instead of 29) when link type is Ethernet. Fixed to use the correct one.

Fixes: fa417f7b ('IB/mlx4: Add support for IBoE')
Signed-off-by: Shani Michaeli <shanim@mellanox.com>
Signed-off-by: Noa Osherovich <noaos@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>