RDMA/cxgb3: Fix iwch_create_cq() off-by-one error

The cxbg3 driver is unnecessarily decreasing the number of CQ entries by
one when creating a CQ.  This will cause the CQ not to have as many
entries as requested by the user if the user requests a power of 2 size.

Signed-off-by: Jon Mason <jon@opengridcomputing.com>
Acked-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>