net/mlx4_en: Fix the return value of mlx4_en_dcbnl_set_all()

mlx4_en_dcbnl_set_all() returns u8, so return value can't be negative in
case of failure.

Fixes: af7d5185

 ("net/mlx4_en: Add DCB PFC support through CEE netlink commands")
Signed-off-by: Kamal Heib <kamalh@mellanox.com>
Signed-off-by: Rana Shahout <ranas@mellanox.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>