NTB: ntb_pingpong: Choose doorbells based on port number

This commit fixes pingpong support for existing drivers that do not
implement ntb_default_port_number() and ntb_default_peer_port_number().
This is required for hardware (like the crosslink topology of
switchtec) which cannot assign reasonable port numbers to each port due
to its perfect symmetry.

Instead of picking the doorbell to use based on the the index of the
peer, we use the peer's port number. This is a bit clearer and easier
to understand.

Fixes: c7aeb0af ("NTB: ntb_pp: 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>