iw_cxgb4: Make function read_tcb() static

Fixes the following sparse warning:

drivers/infiniband/hw/cxgb4/cm.c:658:6: warning:
 symbol 'read_tcb' was not declared. Should it be static?

Fixes: 11a27e21 ("iw_cxgb4: complete the cached SRQ buffers")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Raju Rangoju <rajur@chelsio.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>