USB: fix interface runtime-PM settings

This patch (as1379) reworks the logic for handling USB interface
runtime-PM settings -- hopefully it's right this time!  The problem is
that when a driver is unbound or binding fails, runtime PM for the
interface always gets disabled.  But pm_runtime_disable() nests, so it
shouldn't be called unless the interface was previously enabled for
runtime PM.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Reported-by: Rob Duncan <Robert.Duncan@exar.com>
Tested-by: Rob Duncan <Robert.Duncan@exar.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>