drm/nouveau/secboot/acr: fix memory leak

In case memory resources for *bl_desc* were allocated, release
them before return.

Addresses-Coverity-ID: 1472021 ("Resource leak")
Fixes: 0d466901 ("drm/nouveau/secboot/acr: Remove VLA usage")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>