nouveau/gsp: use correct size for registry rpc.

Timur pointed this out before, and it just slipped my mind,
but this might help some things work better, around pcie power
management.

Fixes: 8d55b0a9

 ("nouveau/gsp: add some basic registry entries.")
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/576336/