NTB: perf: Fix support for hardware that doesn't have port numbers

Legacy drivers do not have port numbers (but is reliably only two ports)
and was broken by the recent commit that added mult-port support to
ntb_perf. This is especially important to support the cross link
topology which is perfectly symmetric and cannot assign unique port
numbers easily.

Hardware that returns zero for both the local port and the peer should
just always use gidx=0 for the only peer.

Fixes: 5648e56d ("NTB: ntb_perf: Add full multi-port NTB API support")
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Acked-by: Allen Hubbe <allenbh@gmail.com>
Tested-by: Alexander Fomichev <fomichev.ru@gmail.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>