nvme: cleanup setting the disk name

Return false from nvme_set_disk_name and let the caller set the
non-multipath name instead of duplicating the naming information in two
places.  Also remove the pointless local variables for the disk name
and flags and the not needed ctrl argument.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Javier González <javier.gonz@samsung.com>