RDMA/iwcm: Get rid of enum iw_cm_event_status

The IW_CM_EVENT_STATUS_xxx values were used in only a couple of places;
cma.c uses -Exxx values instead, and so do the amso1100, cxgb3 and cxgb4
drivers -- only nes was using the enum values (with the mild consequence
that all nes connection failures were treated as generic errors rather
than reported as timeouts or rejections).

We can fix this confusion by getting rid of enum iw_cm_event_status and
using a plain int for struct iw_cm_event.status, and converting nes to
use -Exxx as the other iWARP drivers do.

This also gets rid of the warning

    drivers/infiniband/core/cma.c: In function 'cma_iw_handler':
    drivers/infiniband/core/cma.c:1333:3: warning: case value '4294967185' not in enumerated type 'enum iw_cm_event_status'
    drivers/infiniband/core/cma.c:1336:3: warning: case value '4294967186' not in enumerated type 'enum iw_cm_event_status'
    drivers/infiniband/core/cma.c:1332:3: warning: case value '4294967192' not in enumerated type 'enum iw_cm_event_status'

Signed-off-by: Roland Dreier <roland@purestorage.com>
Reviewed-by: Steve Wise <swise@opengridcomputing.com>
Reviewed-by: Sean Hefty <sean.hefty@intel.com>
Reviewed-by: Faisal Latif <faisal.latif@intel.com>