nouveau/gsp: use correct size for registry rpc.

commit 61712c94 upstream.

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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>