usb gadget: don't save bind callback in struct usb_composite_driver

The bind function is most of the time only called at init time so there
is no need to save a pointer to it in the composite driver structure.

This fixes many section mismatches reported by modpost.

Signed-off-by: Michał Nazarewicz <m.nazarewicz@samsung.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>