ntb_transport: Limit memory windows based on available, scratchpads

When the underlying NTB H/W driver advertises more memory windows
than the number of scratchpads available to setup MW's, it is likely
that we may end up filling the remaining memory windows with garbage.
So to avoid that, lets limit the memory windows that transport driver
can setup based on the available scratchpads.

Signed-off-by: Shyam Sundar S K <Shyam-sundar.S-k@amd.com>
Acked-by: Allen Hubbe <Allen.Hubbe@dell.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>