net/mlx4: Avoid dealing with MAC index in UPDATE_QP wrapper if not needed

The current wrapper implementation of the UPDATE_QP command tries to get
the MAC index, even if MAC wasn't set by the VF. Fix it up to only handle
the MAC field if it's valid.

Fixes: ce8d9e0d

 ('net/mlx4_core: Add UPDATE_QP SRIOV wrapper support')
Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>