nvme-tcp: don't initialize ret variable

No point in initializing ret variable to 0 in nvme_tcp_start_io_queue()
since it gets overwritten by a call to nvme_tcp_start_queue().

Signed-off-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>