IB/core: Fix query port failure in RoCE

IB/core: Fix query port failure in RoCE

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: fad61ad4e755 ('IB/core: Add subnet prefix to port info')
Signed-off-by: Eli Cohen <[email protected]>
Signed-off-by: Leon Romanovsky <[email protected]>
Reviewed-by: Steve Wise <[email protected]>
Signed-off-by: Doug Ledford <[email protected]>