Fix to return a negative error code from the mthca_cmd_init() error
handling case instead of 0, as done elsewhere in this function.

Fixes: 80fd8238 ("[PATCH] IB/mthca: Encapsulate command interface init")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>