net/mlx4_core: Set correctly allow_loopback flag

The allow_loopback flag was wrongly set using arithmetic bit operation, change
the code to use logical bit operation.

Signed-off-by: Hadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>