nouveau/gsp/r535: Fix a NULL vs error pointer bug

The r535_gsp_cmdq_get() function returns error pointers but this code
checks for NULL.  Also we need to propagate the error pointer back to
the callers in r535_gsp_rpc_get().  Returning NULL will lead to a NULL
pointer dereference.

Fixes: 176fdcbd ("drm/nouveau/gsp/r535: add support for booting GSP-RM")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Danilo Krummrich <dakr@redhat.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/f71996d9-d1cb-45ea-a4b2-2dfc21312d8c@kili.mountain

drivers/gpu/drm/nouveau/nvkm/subdev/gsp/r535.c

@@ -689,8 +689,8 @@ r535_gsp_rpc_get(struct nvkm_gsp *gsp, u32 fn, u32 argc)
 	struct nvfw_gsp_rpc *rpc;
 
 	rpc = r535_gsp_cmdq_get(gsp, ALIGN(sizeof(*rpc) + argc, sizeof(u64)));
-	if (!rpc)
-		return NULL;
+	if (IS_ERR(rpc))
+		return ERR_CAST(rpc);
 
 	rpc->header_version = 0x03000000;
 	rpc->signature = ('C' << 24) | ('P' << 16) | ('R' << 8) | 'V';