usb-host-libusb: Configuration 0 may be a valid configuration

Quoting from: linux/Documentation/ABI/stable/sysfs-bus-usb:

	Note that some devices, in violation of the USB spec, have a
	configuration with a value equal to 0. Writing 0 to
	bConfigurationValue for these devices will install that
	configuration, rather then unconfigure the device.

So don't compare the configuration value against 0 to check for unconfigured
devices, instead check for a LIBUSB_ERROR_NOT_FOUND return from
libusb_get_active_config_descriptor().

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
