IB/mlx4: Check gid_index return value

Check the returned GID index value and return an error if it is invalid.

Fixes: 5070cd22 ('IB/mlx4: Replace mechanism for RoCE GID management')
Signed-off-by: Daniel Jurgens <danielj@mellanox.com>
Reviewed-by: Mark Bloch <markb@mellanox.com>
Reviewed-by: Yuval Shaia <yuval.shaia@oracle.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>