usb: gadget: udc-core: fix incompatibility with dummy-hcd

This patch (as1548) fixes a recently-introduced incompatibility
between the UDC core and the dummy-hcd driver.  Commit
8ae8090c (usb: gadget: udc-core: fix
asymmetric calls in remove_driver) moved the usb_gadget_udc_stop()
call in usb_gadget_remove_driver() below the usb_gadget_disconnect()
call.

As a result, usb_gadget_disconnect() gets called at a time when the
gadget driver believes it has been unbound but dummy-hcd believes
it has not.  A nasty error ensues when dummy-hcd calls the gadget
driver's disconnect method a second time.

To fix the problem, this patch moves the gadget driver's unbind
notification after the usb_gadget_disconnect() call.  Now nothing
happens between the two unbind notifications, so nothing goes wrong.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Tested-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>