Currently ib_query_port always attempts to to read the subnet prefix by
calling ib_query_gid(). For RoCE/iWARP there is no subnet manager and no
subnet prefix. Fix this by querying GID[0] only for IB networks.

Fixes: fad61ad4 ('IB/core: Add subnet prefix to port info')
Signed-off-by: NEli Cohen <eli@mellanox.com>
Signed-off-by: NLeon Romanovsky <leon@kernel.org>
Reviewed-by: NSteve Wise <swise@opengridcomputing.com>
Signed-off-by: NDoug Ledford <dledford@redhat.com>