nvme: use nvme_wait_ready in nvme_shutdown_ctrl

Refactor the code to wait for CSTS state changes so that it can be reused
by nvme_shutdown_ctrl.  This reduces the delay between each iteration
that checks CSTS from 100ms in the shutdown code to the 1 to 2ms range
done during enable, matching the changes from commit 3e98c244 that
were only applied to the enable/disable path.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Pankaj Raghav <p.raghav@samsung.com>