Prevent using uninitialized or negative index when handling
steering entries.

Fixes: b12d93d6 ('mlx4: Add support for promiscuous mode in the new steering model.')
Signed-off-by: NTariq Toukan <tariqt@mellanox.com>
Reported-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>