USB: Export if an interface driver supports autosuspend.

Create a new sysfs file per interface named supports_autosuspend.  This
file returns true if an interface driver's .supports_autosuspend flag is
set.  It also returns true if the interface is unclaimed (since the USB
core will autosuspend a device if an interface is not claimed).

This new sysfs file will be useful for user space scripts to test whether
a USB device correctly auto-suspends.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: Oliver Neukum <oliver@neukum.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>