net/mlx4_en: fix error return code in mlx4_en_get_qp()

Fix to return a negative error code from the error handling
case instead of 0.

Fixes: 837052d0 ('net/mlx4_en: Add netdev support for TCP/IP offloads of vxlan tunneling')
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>