usb: gadget: core: fix bug when removing gadget drivers

usb_gadget_disconnect() is responsible of removing
data pullups. Before doing that we must, first, tell
gadget driver we're disconnecting (by calling disconnect
method on gadget driver structure), unbind the gadget
driver and stop the controller.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>