RDMA/cxgb3: Make iwch_poll_cq_one() easier to analyze

Introduce the function __iwch_poll_cq_one() to make iwch_poll_cq_one()
easier to analyze for static source code analyzers. This patch avoids
that sparse reports the following:

drivers/infiniband/hw/cxgb3/iwch_cq.c:187:9: warning: context imbalance in 'iwch_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>