RDMA/srpt: Fix handling of SR-IOV and iWARP ports

Management datagrams (MADs) are not supported by SR-IOV VFs nor by iWARP
ports. Support SR-IOV VFs and iWARP ports by only logging an error message
if MAD handler registration fails.

Link: https://lore.kernel.org/r/20190930231707.48259-10-bvanassche@acm.org
Cc: Honggang LI <honli@redhat.com>
Cc: Laurence Oberman <loberman@redhat.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>