USB: unconfigure devices which have config 0

Some USB devices do have a configuration 0, in contravention of the
USB spec.  Normally 0 is supposed to indicate that a device is
unconfigured.

While we can't change what the device is doing, we can change usbcore.
This patch (as852) allows usb_set_configuration() to accept a config
value of -1 as indicating that the device should be unconfigured.  The
request actually sent to the device will still contain 0 as the value.
But even if the device does have a configuration 0, dev->actconfig
will be set to NULL and dev->state will be set to USB_STATE_ADDRESS.

Without some sort of special-case handling like this, there is no way
to unconfigure these non-compliant devices.

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