RDMA/cxgb4: Call iwpm_init() only once

We need to only register with the iwpm core once.  Currently it is
being done for every adapter, which causes a failure for each adapter
but the first, making multiple adapters unusable.

Fixes: 9eccfe10 ("RDMA/cxgb4: Add support for iWARP Port Mapper user space service")
Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>