net/mlx4_core: Fix access to uninitialized index

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: Tariq Toukan <tariqt@mellanox.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>