mlx4_en: remove unnecessary returned value

The function mlx4_en_arm_cq always returns zero. So change the
return type of the function mlx4_en_arm_cq to void.

CC: Joe Jin <joe.jin@oracle.com>
CC: Junxiao Bi <junxiao.bi@oracle.com>
Signed-off-by: Zhu Yanjun <yanjun.zhu@oracle.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>