net/smc: allow pnetid-less configuration

Without hardware pnetid support there must currently be a pnet
table configured to determine the IB device port to be used for SMC
RDMA traffic. This patch enables a setup without pnet table, if
the used handshake interface belongs already to a RoCE port.

Signed-off-by: Ursula Braun <ubraun@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>