nvme-pci: Fix the instructions for disabling power management

pcie_aspm=off tells the kernel not to modify the ASPM configuration. This
setting does not guarantee that ASPM (Active State Power Management) is
disabled. Hence add pcie_port_pm=off. This disables power management for
all PCIe ports.

This patch has been tested on a workstation with a Samsung SSD 970 EVO Plus
NVMe SSD.

Fixes: 4641a8e6

 ("nvme-pci: add trouble shooting steps for timeouts")
Cc: Keith Busch <kbusch@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Keith Busch <kbusch@kernel.org>