nvme: also skip discard granularity updates in nvme_config_discard

Don't just skip the discard sectors and segments but also the granularity
if a value was already set before.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>