IB/mlx4: Fix reading SL field out of cqe->sl_vid

Commit f780a9f1 ("mlx4_core: Add ethernet fields to CQE struct")
introduced a bug in how wc->sl is set in mlx4_ib_poll_one() -- since
cqe->sl_vid is a big-endian value, the shift must be done after
converting to host endianness.

This bug was found using sparse endianness checking.

Signed-off-by: Roland Dreier <rolandd@cisco.com>