RDMA/cxgb4: Make c4iw_poll_cq_one() easier to analyze

Introduce the function __c4iw_poll_cq_one() such that c4iw_poll_cq_one()
becomes easier to analyze for static source code analyzers. This patch
avoids that sparse reports the following:

drivers/infiniband/hw/cxgb4/cq.c:401:36: warning: context imbalance in 'c4iw_flush_hw_cq' - unexpected unlock
drivers/infiniband/hw/cxgb4/cq.c:824:9: warning: context imbalance in 'c4iw_poll_cq_one' - different lock contexts for basic block

Compile-tested only.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Steve Wise <swise@chelsio.com>
Acked-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>