usb: gadget: Convert drivers to use module_usb_composite_driver()

Use the module_usb_composite_driver() macro where applicable to
eliminate the module_init/module_exit boilerplate in USB gadget composite
drivers.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Felipe Balbi <balbi@ti.com>