USB: add runtime PM for PCI-based host controllers

This patch (as1386) adds runtime-PM support for PCI-based USB host
controllers.  By default autosuspend is disallowed; the user must
enable it by writing "auto" to the controller's power/control sysfs
attribute.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>