PCIe 4.0 defines the 16.0 GT/s link speed.  Links can run at that speed
without any Linux changes, but previously their sysfs "max_link_speed" and
"current_link_speed" files contained "Unknown speed", not the expected
"16.0 GT/s".

Add decoding for the new 16 GT/s link speed.
Signed-off-by: NJay Fang <f.fangjian@huawei.com>
[bhelgaas: add PCI_EXP_LNKCAP2_SLS_16_0GB]
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>
Reviewed-by: NDongdong Liu <liudongdong3@huawei.com>