nvme: make nvme_tls_attrs_group static

To suppress the compiler "warning: symbol 'nvme_tls_attrs_group' was not
declared. Should it be static?"

Fixes: 1e48b34c9bc79a ("nvme: split off TLS sysfs attributes into a separate group")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>