infiniband: remove set but not used variables 'rv'

Fixes gcc '-Wunused-but-set-variable' warning:

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]

It is never used since introduction in bdcf26bf9b3a ("rdma/siw: network and RDMA core interface")

Signed-off-by Mao Wenan <[email protected]>

 drivers/infiniband/sw/siw/siw_main.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)