drm/nouveau: Fix error pointer dereference in r535_gsp_msgq_recv()

If "rpc" is an error pointer then return directly.  Otherwise it leads
to an error pointer dereference.

Fixes: 50f29005 ("drm/nouveau: support handling the return of large GSP message")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: Zhi Wang <zhiw@nvidia.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/b7052ac0-98e4-433b-ad58-f563bf51858c@stanley.mountain