NVMe: __nvme_submit_admin_cmd() can be static

drivers/block/nvme-core.c:865:5: sparse: symbol '__nvme_submit_admin_cmd' was not declared. Should it be static?

Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>