PCI/LINK: Report degraded links via link bandwidth notification

A warning is generated when a PCIe device is probed with a degraded link,
but there was no similar mechanism to warn when the link becomes degraded
after probing.  The Link Bandwidth Notification provides this mechanism.

Use the Link Bandwidth Management Interrupt to detect bandwidth changes,
and rescan the bandwidth, looking for the weakest point.  This is the same
logic used in probe().

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Lukas Wunner <lukas@wunner.de>