nvmet-tcp: constify nvmet_tcp_ops

nvmet_tcp_ops is never modified and can be made const to allow the
compiler to put it in read-only memory, as done in other transports.

Before:
   text    data     bss     dec     hex filename
  16164     160      12   16336    3fd0 drivers/nvme/target/tcp.o

After:
   text    data     bss     dec     hex filename
  16277      64      12   16353    3fe1 drivers/nvme/target/tcp.o

Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: Himanshu Madhani <himanshu.madhani@oracle.com>
Reviewed-by: Israel Rukshin <israelr@mellanox.com>
Acked-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>