nvme: Detect devices that are class Storage Express

This adds support to detect the catchall PCI class for NVMe devices.
It allows the drivers to work with most NVMe devices that don't need
specific detection due to quirks etc.

Tested against a Samsung 960 EVO drive.

Signed-off-by: Jon Nettleton <jon@solid-run.com>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>