IB/mlx5: Fix decision on using MAD_IFC

Fix the condition that dictates when MAD_IFC should be used. According
to firmware specifications, MAD_IFC commands must be used only if the
ib_virt capability is off.

Signed-off-by: Eli Cohen <eli@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>