net/mlx4_en: Fix the return value of mlx4_en_dcbnl_set_state()

mlx4_en_dcbnl_set_state() returns u8, the return value from
mlx4_en_setup_tc() could be negative in case of failure, so fix that.

Fixes: af7d5185

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