drm/nouveau: refine the variable names in r535_gsp_msg_recv()

The variable "msg" in r535_gsp_msg_recv() actually means the GSP RPC.

Refine the names to align with the terms in the kernel doc.

No functional change is intended.

Signed-off-by: Zhi Wang <zhiw@nvidia.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250124182958.2040494-11-zhiw@nvidia.com