nvme: merge nvme_shutdown_ctrl into nvme_disable_ctrl

Many of the callers decide which one to use based on a bool argument and
there is at least some code to be shared, so merge these two.  Also
move a comment specific to a single callsite to that callsite.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Hector Martin <marcan@marcan.st>