NTB: ntb_perf: Disable NTB link after clearing peer XLAT registers

If ntb link disabled before clearing peer's XLAT register, the clearing
won't have any effect since the link is already down. So modified the
sequence so that the link is down only towards the end of the function
after clearing the XLAT register

Signed-off-by: Sanjay R Mehta <sanju.mehta@amd.com>
Acked-by: Allen Hubbe <allenbh@gmail.com>
Acked-by: Logan Gunthorpe <logang@deltatee.com>
Acked-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>