net/mlx4_core: Demote simple multicast and broadcast flow steering rules

In SRIOV, when simple (i.e - Ethernet L2 only) flow steering rules are
created, always create them at MLX4_DOMAIN_NIC priority (instead of
the real priority the function created them at). This is done in order
to let multiple functions add broadcast/multicast rules without
affecting other functions, which is necessary for DPDK in SRIOV.

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>