rdma: Add option to provide "-" sign for the port number

rdma: Add option to provide "-" sign for the port number

According to the IBTA spec [1], the physical connected port is provided
for the QP in RTR-to-INIT stage performed by modify_qp(). It causes
to do not have port number for newly created QPs.

The following patch adds "-" sign to present absence of port, because
QPs are going to be associated with rdmatool link object, which needs
port number as an index.

[1] InfiniBand Architecture Release 1.3 -
	"Table 96 QP State Transition Properties"

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>