IB/mlx5: Fix duplicate const warning

Fixes the following sparse warning:

drivers/infiniband/hw/mlx5/main.c:2574:25: warning: duplicate const

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>