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.

Cc: <stable@vger.kernel.org> # v6.7
Fixes: 8d55b0a9 ("nouveau/gsp: add some basic registry entries.")
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240130032643.2498315-1-airlied@gmail.com