gpu: nova-core: gsp: use () as message type for GspInitDone message

gpu: nova-core: gsp: use () as message type for GspInitDone message

`GspInitDone` has no payload whatsoever, so the unit type `()` is the
correct way to represent its message content. We can use it now that
`()` implements `FromBytes`.

Reviewed-by: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/20251215-transmute_unit-v4-2-477d71ec7c23@nvidia.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>