greybus: es2: Allow proper release of greybus host device

The usb core disables the Interface prior to calling ->disconnect() for
the usb driver. That disallows the es2 driver to remove the greybus host
device and every entity below it in a clean way, as the greybus core may
want to do few operations over the usb connection before getting
removed.

And so we see bunch of errors while we remove the es2 module, like:
"urb cport in error -108 (dropped)"

The usb core has a special per-driver flag, 'soft_unbind', for such usb
drivers. If this flag is set by a driver, the usb core doesn't disable
the Interface prior to calling ->disconnect().

Set that flag for es2.

Tested by removing the gb-es2.ko module on both EVT1.5 and qemu with
gbsim. The interface isn't disabled by the core and the driver is still
able initiate greybus operations over the Interface. This can be
properly tested only after the next patch which removes the greybus host
device before disabling the urbs.
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>

drivers/staging/greybus/es2.c

@@ -1083,6 +1083,7 @@ static struct usb_driver es2_ap_driver = {
 	.probe =	ap_probe,
 	.disconnect =	ap_disconnect,
 	.id_table =	id_table,
+	.soft_unbind =	1,
 };
 
 module_usb_driver(es2_ap_driver);