RDMA/cxgb4: Fix error handling in create_qp()

The variable ret is assigned return values in a couple of places, but
its value is never returned.  This patch makes use of the ret variable
so that the caller get correct error codes returned.

The following changes are also introduced:

- The alloc_oc_sq function can return -ENOSYS or -ENOMEM so we want to
  get the return value from it.

- Change the label names to improve readability.

Signed-off-by: Emil Goode <emilgoode@gmail.com>
Acked-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>