NTB: ntb_perf: Clear stale values in doorbell and command SPAD register

when ntb_perf is unloaded, the command scratchpad register still
retains the last initialized value of PERF_CMD_INVAL. When ntb_perf
is re-loaded and reads peer command scratchpad register and it mis
interprets the peer state as initialized.

To avoid this, clearing the local side command scratchpad register
in perf_disable_service

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>