net/mlx4: Check if Granular QoS per VF has been enabled before updating QP qos_vport

The Granular QoS per VF feature must be enabled in FW before it can be
used.

Thus, the driver cannot modify a QP's qos_vport value (via the UPDATE_QP FW
command) if the feature has not been enabled -- the FW returns an error if
this is attempted.

Fixes: 08068cd5 ("net/mlx4: Added qos_vport QP configuration in VST mode")
Signed-off-by: Ido Shamay <idos@mellanox.com>
Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>