drm/nouveau/pmu/gm20b,gp10b: Fix Falcon bootstrapping

The low-level Falcon bootstrapping callbacks are expected to return 0 on
success or a negative error code on failure. However, the implementation
on Tegra returns the ID or mask of the Falcons that were bootstrapped on
success, thus breaking the calling code, which treats this as failure.

Fix this by making sure we only return 0 or a negative error code, just
like the code for discrete GPUs does.

Fixes: 86ce2a71 ("drm/nouveau/flcn/cmdq: move command generation to subdevs")
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>