IB/ehca: Require in_wc in process_mad()

If the caller does not pass a valid in_wc to process_mad(), return MAD
failure status, as it is not possible to generate a valid MAD redirect
response (and redirects are the only MAD responses ehca generates).

Signed-off-by: Alexander Schmidt <alexs@linux.vnet.ibm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>