nvme-pci: add trouble shooting steps for timeouts

Many users have encountered IO timeouts with a CSTS value of 0xffffffff,
which indicates a failure to read the register. While there are various
potential causes for this observation, faulty NVMe APST has been the
culprit quite frequently. Add the recommended troubleshooting steps in
the error output when this condition occurs.

Signed-off-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>