nvme: remove redundant validation in nvme_start_ctrl()

We've already validated the 'kato' in nvme_start_keep_alive(), thus no
need to validate it again in nvme_start_ctrl(). Remove it.

Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>