drm/nouveau/secboot: Make acr_r352_ls_gpccs_func static

Fix sparse warning:

drivers/gpu/drm/nouveau/nvkm/subdev/secboot/acr_r352.c:1092:1:
 warning: symbol 'acr_r352_ls_gpccs_func' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>