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

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 configuration structure.

This fixes many section mismatches reported by modpost.

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