RDMA/siw: Remove unnecessary kthread create/destroy printouts

There is already a warning if we cannot start any thread, and stopping
those threads is not worth spamming the console.

This also corrects a warning from gcc:

 drivers/infiniband/sw/siw/siw_main.c: In function 'siw_create_tx_threads':
 drivers/infiniband/sw/siw/siw_main.c:91:11: warning:
  variable 'rv' set but not used [-Wunused-but-set-variable]

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Bernard Metzler <bmt@zurich.ibm.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>