net/mlx4: Fix the check in attaching steering rules

Our previous patch (cited below) introduced a regression
for RAW Eth QPs.

Fix it by checking if the QP number provided by user-space
exists, hence allowing steering rules to be added for valid
QPs only.

Fixes: 89c55768 ("net/mlx4_en: Avoid adding steering rules with invalid ring")
Reported-by: Or Gerlitz <gerlitz.or@gmail.com>
Signed-off-by: Talat Batheesh <talatb@mellanox.com>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Acked-by: Or Gerlitz <ogerlitz@mellanox.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>